買ってきます vs 買ってます

I'm going through Genki 1. I think 買ってきます is "I will buy", but I don't know why. I don't remember seeing this form before with き added, so I have no clue what this about. Did I just forget something? I know つもり indicates your intention to do something or "I will". Please help.

This is the 〜てくる form, with 来る/"to come" (typically written in hiragana in this case) being used as an auxiliary verb.

It literally means, "I'll buy (something) and come back", and is the most common way to express in Japanese what we would say in English as "I'll go buy (something)."

(In other cases, it can also cover the meanings of "start to (do something)" e.g. 雨が降ってきた or something occurring in your direction, e.g. 猫が飛び出してきた)