What does "it" in "If it wasn't for Amber..." refer to?

If it wasn't for Amber he wouldn't be able to marry Claire.

Please, what does "it" in this sentence refer to?

The pronoun "it" refers to the actions of Amber. Some linguists would call that a dummy pronoun. Informally it can be unrolled like this:

If it wasn't = If it was not = Without

...and means much the same as:

Without Amber he wouldn't be able to marry Claire.

But "if it wasn't" is a shade different, in that it gives more emphasis to chance, rather than just stating a logical necessity. Example:

Without eggs we can't make meringue. (necessity)

vs.

If it wasn't for pick-pockets I'd have no sex life at all. - Rodney Dangerfield (chance)