This contraction stands for:

Note that this is an extremely common contraction* which is even used in speech when it's not contracted in writing. In other words, when reading aloud, most people will read "He is all I have" as He's all I have because it sounds more natural and makes for a more idiomatic rhythm.


*The forms -'s and -s' are also added to nouns to show possession, but we won't treat them here because these are suffixes, not contractions.

it's vs its


Care must be taken to distinguish it's meaning it is or it has from the possessive form its = belonging to it:

It's raining cats and dogs.

It's taken three weeks to paint this.

A dog always knows its owner's voice.

who's vs whose


The contraction who's is used for who has and who is, whereas whose is the possessive form of who

Who's in the house?

Who's bought the house?

Whose house is this?