Are there catholics? what is the difference between the mediator and the intercessory?

let's say Mary is a mediator with Jesus? is it true all saints are mediators because their goal is helping us to reach God? well my priest told me that even he doesn't explain me so much about the verse of the letter of timothy where it says the only mediator between God and humans is Jesus christ man, so if Jesus christ is God too then how can be my mediator too?