You're right, these posts assume one knows what the Y combinator is for. Wikipedia has a decent explanation: https://en.wikipedia.org/wiki/Fixed-point_combinator

Key point:

    the Y combinator implements simple recursion. In the lambda calculus it is not possible to refer to the definition of a function in a function body. Recursion may only be achieved by passing in a function as a parameter. The Y combinator demonstrates this style of programming. 

You don't really need it in modern programming languages that natively support named functions and recursion, but it's an interesting part of the underlying theory of lambda calculus.