What does a zero after a decimal point mean?

A zero after a decimal point means that you need to figure out the significant figures.
Say, for instance, you have 38.0
This means that you have 3 significant figures, 3, 8, and 0.
So if you were to add it to another number with 3 significant figures, such as 2.00, you would then have to make sure that you have 3 significant figures in your answer. So, instead of saying 38+2=40, you have to say 38.0+2.00=40.0

This is kind of confusing if you aren't learning it in school. Your teachers can explain significant figures a lot better than I can. Hope this helps you, though!