A variable is a named box in which a data value can be stored. The variable itself isn't the data value, just like a carton of milk is not the same thing as milk (it's waxed cardboard and such containing milk).
On the other hand, if somebody needs milk and you hand them a full milk carton, they're not going to complain, and the same goes for Rust. If a Rust expression needs an integer, and we provide a variable containing an integer, Rust will be perfectly happy with that.
Variables are most often created using the let keyword:
let x = 10;
This statement creates a variable called x containing the 10 value in it. Once that's done, we can refer to x as part of the expressions. For example, x + 5 is now a valid expression, with a resulting value of 15.