it means "that is correct", "that is true", "that is appropriate", "that is good" you can use it to express agreement, approval, or excitement And if you're looking for something, it means "that is the thing I'm looking for"...

This can mean many different things depending on the context. For example: If you're explaining something or telling a story, 'That's it' can mean 'That's all there is to say' or 'It's finished'. If you are looking for a thing or an idea and you or someone else finds it, it can mean 'That's what I was looking for/That's was I was thinking of'. If you can give us the context, or a few lines of dialogue, we can explain exactly what it means in that situation.