Rust's first assumption about any variable you create is that it is immutable by default. If you say let x=1;, the variable x is bound to the value 1 for the lifetime of the program. Any variable that might change has to be specifically declared as such with the mut (for "mutable") keyword; e.g., let mut x=1;.
Immutable entities in a program are easier to reason about and make guarantees about- especially guarantees about memory safety, which is Rust's big selling point. If the compiler knows some element in a given scope doesn't change once it's assigned, most of the memory management issues around that element evaporate.
This "Sir, may I?" strategy is a deliberate design choice for Rust. It forces the programmer to think about what actually does need to change in a program. Taking input from the user, for instance, or reading from a file, are operations that must be handled mutably.
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