First, recognize they are two words that you can live your entire life without every hearing (in fact I never have, heard them spoken that is) or reading. So don't sweat them. It's as if learning English weren't hard enough, without subjecting its students to acamedic vocablulary!
Lexis is the lexicon of a language, all its words and idioms.
Collocation is the arrangment of, in grammar terms, a sentence.
- For Teachers