If we just had nouns, things would get dreadfully repetitive! Here's such an example:
Bob dropped his ball. The ball rolled away. His mum picked the ball up.
Try to stay awake! What we need is an alternative word for ball, once we know what it is. Aha - it.
Bob dropped his ball. It rolled away. His mum picked the ball up.
The word it is what's known as a pronoun - something that can be used instead of a noun in a sentence.