fibonacci series pseudocode

Question

Hey everyone,
Could someone help me with the above mentioned. I understand that the series means:
each number is the sum of the previous 2 e.g
0 1 1 2 3 5 8 13...so
5+8 = 13
3+5 = 8 etc
now I came across a numbers of pseudocode examples which I didnt understand so cud u plz explain step by step
Thanks.