It is an impersonal pronoun. It has several uses.

1. To refer to lifeless things and animals:

I have bought a chair. It is made of teak wood.
The dog is barking. It must be hungry.

I bought a new mobile phone last week. It isn’t working.

2. To refer to a young child, except when the sex has to be made explicit:

The baby cried because it was hungry.

3. As a provisional subject at the beginning of a sentence. In this case, it represents the real subject which follows later.

It is useless to wait any more. (=To wait anymore is useless.)
It is true that I wanted to marry her. (=That I wanted to marry her is true.)

4. To refer to some statement in the preceding clause:

He is really responsible for this problem; and he knows it.
He was committing a mistake, and he didn’t know it.

5. As an impersonal pronoun:

It is raining.
It is getting dark.

6. To refer to a collective noun when it is viewed as a single unit:

The committee decided to hold its next meeting on the 25th.
The jury gave its unanimous verdict against the prisoner.

7. It can also be used as a preparatory subject when the real subject is a to-infinitive.

It is easy to find fault with others. (= To find fault with others is easy.)