As always with Nietzsche, it is worth looking at the context of the quote. This sentence is part of the first metamorphosis of the "three metamorphoses of the spirit" and in order to understand the quote, you have to understand what it stands for (translation from the Cambridge Edition, 2006):
On the Three Metamorphoses
Three metamorphoses of the spirit I name for you: how the spirit becomes
a camel, and the camel a lion, and ﬁnally the lion a child.
To the spirit there is much that is heavy; to the strong, carrying
spirit imbued with reverence. Its strength demands what is heavy and
What is heavy? thus asks the carrying spirit. It kneels down like a camel
and wants to be well loaded.
What is heaviest, you heroes? thus asks the carrying spirit, so that I
might take it upon myself and rejoice in my strength.
Is it not this: lowering oneself in order to hurt one’s pride? Letting
one’s foolishness glow in order to mock one’s wisdom?
Or is it this: abandoning our cause when it celebrates victory? Climbing
high mountains in order to tempt the tempter?
Or is it this: feeding on the acorns and grass of knowledge and for the
sake of truth suffering hunger in one’s soul?
Or is it this: being ill and sending the comforters home and making
friends with the deaf who never hear what you want?
Or is it this: wading into dirty water when it is the water of truth, and
not shrinking away from cold frogs and hot toads?
Or is it this: loving those who despise us, and extending a hand to the
ghost when it wants to frighten us?
All of these heaviest things the carrying spirit takes upon itself, like
a loaded camel that hurries into the desert, thus it hurries into its
All these are about how the spirit endures burdensome tasks (burdens itself...like a camel - a beast of burden - is burdened with heavy loads), asking which of them is the most burdensome one.
If you rearrange the sentence, it becomes more clear what is meant:
Wading into the water of truth, even if it is dirty, even if there are cold frogs and hot toads?
It should be clear how wading into dirty water by itself is burdensome, but what about "cold frogs" and "hot toads"? Cold frogs is straightforward: Frogs' skin feels cold and many people do not exactly deem it a pleasant experience to have to touch or be near a frog (think of the Grimm's tale The Frog King, or Iron Heinrich). Hot toads is a bit more tricky: The common (European) toad - the species Nietzsche most probably was acquainted with under the name "toad" - has a slimy secretion on it which, on contact, causes the skin to burn, thus making it a "hot toad".
Thus, the dirtiness, the cold frogs, and the hot toads are all poetic devices to describe just how unpleasant it is to "wade into the water of truth" - since the truth often is veiled by lies and "social practices" and unpleasant to encounter. But, the enumeration suggests, it is necessary for the metamorphosis towards "the spirit of a child" - the standard poetic description of a truly philosophical spirit.
One has to keep in mind that all these tasks that make the spirit "kneel down like a camel" are descriptions of Christian virtues of renouncement of one's desires (resisting temptation, modesty, not to be a burden to somebody, truthfulness, Christian charity), all of which lead us into a (spiritual) desert (a world bereft of all this-worldly satisfaction, containing nothing), and which we have to overcome if we are ever to transform into lions (nihilists, critics of all values, fighters against the status quo - but without the ability of positive determination):
My brothers, why is the lion required by the spirit? Why does the beast
of burden, renouncing and reverent, not sufﬁce?
To create new values – not even the lion is capable of that: but to create
freedom for itself for new creation – that is within the power of the lion.
or even children, who have left all (conventional, authoritarian) determination ("the world") behind, went through the (void, undetermined) desert and now start to create their own values, satisfactions (saying yes means agreeing to something, bringing something to fulfilment), and rules in pure self-determination:
The child is innocence and forgetting, a new beginning, a game, a wheel
rolling out of itself, a ﬁrst movement, a sacred yes-saying.
Yes, for the game of creation my brothers a sacred yes-saying is required.
The spirit wants its will, the one lost to the world now wins its own world".
Thus, the three stages are living the (spiritual/rational - not Epicurean/hedonistic) values, renouncing these values (the ratio/desire dichotomy embodied in "thou shalt", ie. imperatives), and creating one's own values. In other words: (Understand what it means to) be a Kantian/Stoic, (understand what it means to) be a nihilist, (understand what it means to) be an Epicurean, in that order (and without forgetting/seizing to be the former). Mind, it is about metamorphoses, ie. you transform into something different without seizing to be the former: you cannot be a true nihilist without knowing, understanding, and living (in) imposed imperatives since that is what teaches you to endure the sometimes grief consequences, and you cannot be a true Epicurean without living, fighting for and against, and (re-)determining your own imperatives/values, which only really are your own (and not someone else's) if you know what the conventional values are and have learned to endure the consequences of living values even if it sometimes is burdensome, and how to dissociate oneself from the will and values of others.