It still is just numbers and operators (like plus signs) just like before. The only difference is that now more operators are being added and letters can stand in for numbers that change depending on what's going on. The reason people do this is because you aren't necessarily looking at a static situation where numbers aren't changing. Finding one definite answer is only so useful since it only represents one situation.

Let me show you with a simple example. Suppose you have a gift card for $100 at a game shop. You want to get the biggest number of games you can with that $100, so you need to keep track of how many more games you can choose at this moment. How do you do that? Just add up the prices of the games in your hand and take that away from 100, right? Well, you can represent that exact command, "Count the total price of all the games I have in my hand and take that out of my gift card balance to see what I have left" with this:

y = 100 - Σx

The "y" there is just your current amount of money left on your card if you buy all the games you currently have, the Σ is an operator saying "add up a sum", and "x" is the thing to add up (the price of each game). Written in a more familiar form, the above is:

Money Left = $100 - (Game 1 + Game 2 + Game 3 ... + Game N)

where "N" is just the total number of games in your hand. It's the same thing you've always been doing, only now you're representing that you might put a game back or pick another one up (which would change the values of "y" and "x"). It

*looks* alien and bizarre, but you actually do this kind of thing all the time.