Infer / deduct the type from the other types of other variables and from the operation on the variable.
"guess the type" is probably not a good methaphore as there is no guessing in it.
fn main() {
let y: i8 = 42; // explicitely set the type
println!("{y}");
let x = 42; // defaults to i32
println!("{x}");
let z = 42; // at first Rust will assume this is i32, the default
let result = y + z; // When Rust sees this it will understand that z participates
// in a operation where both operands have to be the same type and the other operand
// was explicitely set to be i8. So Rust infers that z is also of type i8.
// It also infers that "result" will be of type i8.
println!("{result}");
}