You write things like 2 + 3 = 5 because you solved 2 + 3 and the answer was 5. You will use the equal sign in both directions to solve problems in math.
If you know that 2 + 3 = 5, then you can solve x + 3 = 5 (x = 2!).
If you know that 2 + 3 = 5, then you can also solve 2 + x = 5 by subtracting 2 from both sides. Then, x = 3.
Doing things to both sides. The equals sign says that both sides of the equation have the same value. If that is true, then you should treat both sides the same so the equality remains. We like to joke that you give the same presents