Loosely speaking, it’s a type signature for a function that 1. takes an immutable reference to a vector of strings and 2. returns an immutable reference to a

Loosely speaking, it’s a type signature for a function that 1. takes an immutable reference to a vector of strings and 2. returns an immutable reference to a string, 3. with the constraint that the strings in the input vector must not be freed before the result is freed. It’s a reasonably concise way of saying all that.