x and y are the letters often used for "variables", just something to take the place of a number, until you know what the number is. They are placeholders. Any letter can be used. x and y are the most common.
So if I know apples cost $2 a pound, I can calculate the cost of apples, in dollars, by the equation cost = 2x, where x is the pounds of apples. so if i buy 3 pounds i pay 2 times 3 = 6 dollars. But if I buy 10 pounds, the cost is 2 times 10 = 20 dollars. x is taking the place of the number of pounds.
x + y =
means to you have some number x, and you ADD (+) another number y, and you get the answer (=)