Why did the programmer quit his job?
Q: Why did the programmer quit his job?
A: Because he didn’t get arrays.
This joke is another programmer joke. "Arrays" are a type of data storage that programmers use to store multiple pieces of similar types of data. To "get" something in colloquial American English can mean to understand it. So, if the programmer did not understand arrays, and he were expected to use them in his job, he would probably want to quit, because his job would be very hard.
On the other hand, the words "get arrays" sounds identical to the words "get a raise", and to "get a raise" is to receive an increase in the amount of money that you are paid to do work. People who work like to periodically make more money for doing their job, and if someone expected this to happen and it did not, they might leave their job over that.
So, this is a pun. If the jokester says it aloud, the person listening to it might hear either or both interpretations of "get arrays", and therein lies the humor.