Kant wrote in his first critique:
Space is not a discursive, or as one says, general concept of relations of things in general, but a pure intuition.
This is simply saying we shouldn't confuse the immediate experience of space with the concepts that we use to talk about it; this actually has been important in both physics and geometry, especially because of the popularity of the Cartesian notion of describing space, where one imposes a system of axes and then gives the coordinates of space; instead, when we look at space we see no cartesian grid, taking this cue leads to the notion of general covariance in physics, and describing geometry intrinsically.
it follows from this an a priori intuition (which is not empirical) underlies all concepts of space.
He's elaborating here what he means by a pure intuition - it's an 'a priori intuition'.
Similarly, geometric propositions, that, for instance in a triangle two sides together are greater than the third, can never be derived from the general concepts of line and triangle, but only from intuition, and indeed a priori with apodictic certainty (A24-5/B39-40)
This is where Kant opens up the possibility for non-Euclidean geometry; if we exchange the axiom he mentions with a similar one (that is easier to work with, and changes nothing in what Kant wrote): that the angles of a triangle need not add upto 180 degrees; then, if they add up to less, we get hyperbolic geometry, and if they add upto more, we get elliptic geometry.
Gauss was known to have read Kants first critique where this extract is taken from (at least five times, according to one source) then one could conjecture that this - which is talking about geometry, his speciality - opened up for him the possibility of making a definite mathematical model of non-Euclidean geometry. Sometimes in mathematics all one needs is a hint or a cue, and Kant may, and more than likely to, have provided this for him.