It's warm in Beijing today.

It has no meaning, and is often used as a subject with no specific meaning. It appears in sentences indicating weather, climate, temperature, time, place and distance.

It is also often used to express the general situation. Too bad-I have so much work that I don't know where to start.

Represents something mentioned before. It can also be used when the speaker does not know or needs to know the gender of the speaker.

Related examples:

1, this is really good, isn't it?

This is really good, isn't it?

Sounds like a good idea. Go!

That sounds like a good idea. Try to achieve it!

3, effective, at least I think it is effective.

It works. I think it works anyway.

Liz wanted to play a joke, but it didn't work out.

Liz wanted to tell it as a joke, but it didn't work.

Take it to a bright place so that I can see it.

Take it to a bright place so that I can see it.