Here's the whole secret of algebra in one line: a letter is just a number we haven't decided on yet. When you write 2x + 3, the x is a placeholder. The moment you say "let x be 3", you can swap the number in and the expression collapses into a single value.
Think of It as a Machine
The toy up top is a little machine. You feed a number into the x slot, the expression does its thing — multiply by 2, then add 3 — and a number pops out the other end. Slide x to 3 and out comes 9. Slide it to 5 and out comes 13. The expression hasn't changed; only the number you fed it has. That's all a letter ever does — it holds a spot open for whatever number you choose.
Why Bother with Letters at All?
Because a letter lets you write a rule once that works for every number. "Double it and add 3" is a sentence; 2x + 3 is the same idea written so cleanly that a calculator, a phone, or a spreadsheet can run it. One short expression covers infinitely many cases — you just decide what x is each time.
2x Means 2 Times X
One thing to lock in early: when a number sits right next to a letter, like 2x, it means 2 multiplied by x. We just drop the times sign to keep things tidy. So 2x with x = 4 is 2 × 4 = 8. We'll lean on that constantly.