used as subject or object:
that thing, animal, etc creature, or group
I saw the house and noticed that it was very old
She had a baby but lost it
the person in question
Who is it?
used as subject of an impersonal verb
It's raining
It's not far to London
used as anticipatory subject or object of a verb
It's no fun being a secretary
I take it that you refuse
used to highlight part of a sentence
It was the President who arrived yesterday
used with many verbs and prepositions as a meaningless object
Run for it
We legged it back to camp
used to refer to an explicit or implicit state of affairs
How's it going?
that which is available
We have one boiled egg and that's it
the important or appropriate thing
Yes, that's just it
the appropriate thing
A bit tighter; that's it