The letters such as x and y are technically speaking just symbols to represent a variable. They could be anything from a to z, and what they actually meant depends on the context of the question.
For example, in this question:
"Alisa bought x apples and y bananas.
How many fruits did she bought in total?"
x will mean the number of apple bought and y will mean the number of banana bought.
For x^2 + y^2 = 13, x and y just represent certain variables, and nothing more than that.