This is an expression, so each word as lost it's individual meaning to meaning something else together. But if you want to dissect it, please note that we construct sentences like this : "subject + verb + complement". But if there is a pronoun it's " subject + pronoun + verb". So in this case, for example, "I love you" would be "je t'aime" (I you love). Weird, I know. Back to your example, "en" is a pronoun. ("en" as several meaning though, it can mean "in" indeed, but not here). This pronoun replace "the thing we were discussing before". "prie" is the verb "prier" to pray. So all together it's kind of "I pray you to do it" but in fact it's just a begging way to say "please" or an elegant way to say "my pleasure". ex :- Venez avec moi, je vous en prie ! (Please come with me) - c'est très généreux. (it's very generous) - Je vous en prie, ça me fait plaisir (It's nothing, my pleasure) - Puis-je ? (Can I ? ) - Mais oui, je vous en prie (of course, please do !)

je vous en prie as i know is like please in english !