In mathematics, every result known descends from something else: it is proven to be true from other facts.
The one exception is axioms: these things we choose to accept without proving them.
We have to choose some axioms, since we cannot prove anything with nothing, but we try and make them as simple and obvious as possible.
For example, Euclidean geometry rests on five axioms, the first of which is "given two points on a plane, it is always possible to construct a straight line passing through these two points". Another states that it is possible to draw a circle with any center and radius.
Using these simple statements, Euclid then proceeds to prove more complex properties of figures on the plane.