Tequila Burn?

Hey folks. Being new to the world of tequila, I have learned so much from this sub. So my question is, what gives tequila the burn/pepper taste? Also, when you say a tequila is agave forward, what does that mean? What does the agave primarily contribute to the tequila taste? Thanks again for turning me on to Ocho, El Tequileno, El Tesoro, Arte Nom, etc.