Is it's a plural noun

The form it's (with apostrophe) is a contraction, a shortened form for the pronoun it and the verb is.

The pronoun it is a singular pronoun, a Word that takes the place of a noun for one thing.

The contraction it's functions as the subject and verb (or auxiliary verb) of a sentence or clause.

it = pronoun

is = be verb

Example: It is a nice day. Or: It's a nice day.