Programs must allocate memory and free memory after they stop using them. Some languages (C) require you to do that stuff manually, others (Go) have a running routine that inspects your memory at runtime in order to identify pieces of memory that aren't being used anymore and cleans those automatically. Rust has invented a mechanism that allows you to write code in such a way that the compiler knows, at compile-time, when every bit of memory is not being used anymore, so it can automatically free those without having to run a process that inspects everything all the time, which can be slow.