Very TL;DR version: a variable has an owner. If you pass it off to another function, you no longer own it and can't use it until/unless it gives the variable back. Rust can be really strict on making sure you aren't trying to use something you don't own at that time. The documentation explains it better than this (and I wrote a longer post but accidentally closed the window and lost it). See also mutability and lifetimes for some pain points people might not be used to.