When is a chemical equation considered balanced?

The reason why Chemical equations are balanced is because each side of the equation must have the same amount of atoms, it is impossible to, for example, have this side with 3 O and that side with 4 O.

How to check:

Here's a really good and famous example(Guranteed your cehmistry teacher will mention this some day!):

CaO + H2O = Ca(OH)2

Is it balanced?

There's one Ca(Calcium) on one side, same with the other side. (Balanced!)

There's 2 Oxygen on one side, same with the other.(OH)2 Means two Oxygen, two Hydrogen.(Balanced!)

There's 2 Hydrogen on both sides as well(H2).

So the whole thing's balanced!

If it's not balanced, you can't change the numbers, you have to change the number in front. e.g. in the chemical equation of photosynthesis, there's a 6H2O. That means that there is 12 hydrogen and 6 Oxygen, change that "6" at the front to balance stuff. If there's no number at the front, that's the same as "1".