What is the meaning of -aí?

Ending of the plural of certain nouns.

-er (person or thing that does an action indicated by the root verb)

-ist (one who follows a particular ideology; professional, amateur, or enthusiast; person who uses something)

-ee (person to whom, or a thing to which, an action is done; person who, or a thing that, does an action; person who is the recipient party to a contract)

Added to nouns to form adjectives.

Suffix used to form verbal nouns

Source: wiktionary.org