Someone who can help translate this code

Question

func fib(n: Int) -> Int {
return n < 2 ? n : (fib(n-1) + fib(n-2))
}
print(fib(3))

Answer 1

The function calls itself recursively, until n is equal to 0 or 1, at which point the recursion ends.
return n < 2 ? n : (fib(n-1) + fib(n-2)), means if n is less than 2 return n, otherwise calculate (fib(n-1) + fib(n-2))

Lets consider the example when n=3 (fib(3))

1. checks n=3 < 2 nope, calculates fib(2) + fib(1)
2. calls fib(2)
3. checks n=2 < 2 nope, calculates fib(1) + fib(0)
4. calls fib(1)
5. checks n=1 < 2 yes, returns 1
6. calls fib(0)
7. checks n=0 < 2 yes, returns 0
8. calls fib(1) (from point 1.)
9. checks n=1 < 2 yes, returns 1

According to the recursion:
fib(3) = fib(2) + fib(1) = fib(1) + fib(0) + fib(1) = 1 + 0 + 1 = 2

Answer 2

The answer above is very explicit and clear. Thank you @Paruyr Danielian.
The function is a famous fibonachi's series.
Can someone tell me the use in real life of this function.