Help with Shop

Hi, I'm a very new player and this is my first time posting on here. I get that items are necessary, but often the way to get them confuses me. I'm mostly playing AI games atm so it's not the biggest problem, but I have seen people get noticeably annoyed or frustrated when I "full-buy" something. What does this mean? Also, when I look up guides for champions, there's a section on items and it has the "recipe price". What does that mean? Thanks for taking the time to answer the questions.