Can someone explain in a simple way what "reducible" mean in complexity theory?
The most common notion of reducibility is many-one polynomial time reducibility.

A problem $A$ is many-one polynomial time reducible to a problem $B$ if there is a polynomial time function $f$ such that $x \in A$ iff $f(x) \in B$.

This definition implies that if $B$ is solvable in polynomial time, then so is $A$.

Other notions also exist, but at this stage you're probably better off concentrating just on this one, since it is the one used to define NP-completeness.