It -- as subject of a sentence

The Third Person It as Subject of a sentence

It as a subject of a sentence can be used in two different ways.

1). As an empty or meaningless word in expressions that concern the time or the weather:

It is one o'clock.
It is raining.
It will soon be supper time.
It is frosty.

2). As the formal subject of a sentence, referring to the deferred real subject that follows the verb:

It is useless to wait. (deferred subject - to wait)
It is a pity that you must go. (deferred subject - that you must go)

In sentences where it refers to a deferred subject, the latter can be moved to the normal position preceding the verb;

To wait is useless.
That you must go is a pity.

Below is a link that further explains usage of It in simple language.
Using It