En is a pronoun, it stands for the thing you were discussing just before. Ex : - Marie a été acceptée à l'université. - J'en suis ravie ! (en = que Marie ait été acceptée à l'université) But it's also just an expression, no need to put too much though into it. It's just something we say that sort of mean "You're welcome/my pleasure" or "please".

Bayane, si je dis "je t'en prie ne pars pas" -- je t'en pris = please.

Je te prie de.... = Please Je t'en prie! = You're welcome! Be carefull ;)