What does y mean in pre-algebra?

Answer 1

In pre-algebra, y is a variable. It holds the place of number that we may not know just yet.

Answer 2

y is a variable just like x is a variable. So what's a variable, right? It's a symbol we use when we don't know how many we have of something or when the number of them can change.

y + 5 means we add 5 to whatever we decide y stands for. It could be any number, so we're just holding its place with the variable.