A Mutable Reference Gives Temporary Permission To Change A Value

A shared reference like &String or &str only allows reading but to modify a borrowed value, we need a mutable reference written as &mut T in source

fn append_status(message: &mut String) {
    message.push_str(" [processed]");
}
 
fn main() {
    let mut message = String::from("Server started");
 
    append_status(&mut message);
 
    println!("{message}");
}

So, here

let mut message = String::from("Server started");

the owner allows the value to be changed (we learned about it in first article)

then, we create a mutable reference

&mut message

finally, the function receives temporary write access

message: &mut String

The function still does not own the String. Once the mutable borrow ends, the original owner can use the modified value again.