It really means that the total amount of energy in the universe is a fixed number, and doesn't change.
So if you're watching what's going on in some fixed bottle, box, pot, or other volume, 'conservation' means that no energy can magically appear in the bottle or disappear from it. The total amount of energy can't change. The energy can only move from one place to another. So if you see the amount of energy change inside there, then you know that either some of the original energy must have leaked out and escaped, or else more energy came in from somewhere.
This is actually a very powerful thing to know. It helps solve a lot of problems.
