A mutable variable is one whose value can be edited after the assignment, whereas an immutable variable is one whose value can’t be edited after the assignment. For declaring a mutable variable, you can use the mut keyword:
let mut x = 5;​
Since x is mutable, its value can be changed later in the program by assigning it a new value.
For declaring an immutable variable, you can omit the mut keyword and just declare the variable as
let y = 20;​
