Y is just a symbol.

Start with a series of numbers. It can be simple:

1,2,3,4,5 . . .

Or if you feel a little more adventurous, it can be a little more complex:

1, 4, 16, 64, 256 . . .

Maybe you choose to write whatever number comes to mind. Maybe you have darts and decide to throw them at the wall with numbers on it. Any way you decide to do it, after a while, it will become boring.  As you are probably aware of, there are ways to represent a series, finite or infinite. Being new to the whole blogging math thing, I still suck at trying to write formulas online. Bear with me. Or bare with me, I don't mind.
In order to work with this formula, you need a series that uses a pattern you can figure out. The second example is one I'll use for discussion. You can figure out the pattern of a series of random numbers, it just involves probability and that's a discussion for another day.
Ok, the series above is simple as 4ⁿ where n is any number greater than or equal to zero. Use any number, like 20, and you will get what that number is in that position is in the series,  1099511627776.
You can go on for ever. Figure out 4 to the power of googol is, for all I care. Or we just say that any answer you get is y.

4ⁿ = y

The point I'm trying to make isn't series are fun, but letters are meaningless. Y is a symbol that can be given rules and reason, whether it's mathematical, or I'm texting dumb people with "y u so dumb?"
Symbols gain their power by some from an all powerful, all knowing book (The Dictionary!) or most people give them power by context.