f(n) is the nth term
Each term f(n) is found by adding the terms just prior to the nth term. Those two terms added are f(n-1) and f(n-2)
The term just before nth term is f(n-1)
The term just before the (n-1)st term is f(n-2)
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For example, let's say n = 3 indicating the 3rd term
n-1 = 3-1 = 2
n-2 = 3-2 = 1
So f(n) = f(n-1) + f(n-2) turns into f(3) = f(2) + f(1). We find the third term by adding the two terms just before it.
f3) = third term
f(2) = second term
f(1) = first term
