Hello Ram,
Axiom means something which is true. There is no proof of it.
For example - 2 comes after 1 is universally true, so it is an axiom.
Meanwhile, a theorem is something which can be proved, like Basic Proportionality Theorem
To leanr about Euclid's Axiom, please visit https://www.teachoo.com/subjects/cbse-maths/class-9th/ch5-9th-introduction-to-euclid's-geometry/