Adjust Seasoning are instructions to check the taste of a dish, to see if it is to your taste, and if not, to adjust the taste. Adding additional flavourings is inferred, because of course flavouring cannot be removed from a dish — though an excess of one flavour can partly be masked or balanced by another flavour.
The instructions to “Adjust Seasoning” are almost always given at the end of preparation. Sometimes this means just before serving; othertimes, it means just before putting something into the oven to bake.
Before you adjust the seasoning on something, it is imperative that you taste it first. In doing so, you should use a fork or spoon that you are not then going to use to stir the ingredients with. You never, though, taste uncooked items which are unsafe to taste when raw (such as sausage-meat, for instance.)
The most usual items used to Adjust Seasoning are salt and pepper, but they can also be herbs, spices, an acid, etc.
Liquid items will reduce in volume during cooking, intensifying the flavour, so you adjust the seasoning for them at the end of cooking.