Took me a while to get it

i dont get ti

They're adding all the numbers that start with 90 (ex: 91,92,93) and getting the sum.

Sum is the mathematical term for the answer to an addition problem. Look at the top numbers which say "90+91+92+93+94+95+96+97+98+99" the joke is the person summed(added) up the 90's.