C++ programming question

Please help and explain

// Implement function fibonacci that returns the nth Fibonacci number. // Fibonnaci numbers can be computed as the sum of the preceeding two. // You will find it useful to keep track the two previous Fibonacci // numbers. This is harder than it looks. // // fibonacci(1) returns 1 // fibonacci(2) returns 1 // fibonacci(3) returns 2 // fibonacci(4) returns 3 // fibonacci(5) returns 5 // fibonacci(6) returns 8 // int fibonacci(int n) { // TODO: Complete this function return -999; }