They are party affiliated people, paid by the county, to ensure the proper operation of the polling place, reviewing ballots for possible challenge, documenting challenges, counting them and reporting the results to the county clerk.
They do not check identities or hand out ballots themselves, but if they see any improprieties in that sector, they report it to the county clerk who informs representatives of both parties and sends the sheriff to straighten things out if need be.
The terminology is sometimes used to refer to people on the outside of the polling places who report who arrives at the polls, asks questions if the voter will cooperate, and report to the party headquarters. Their function is to notify party headquarters who shows up so the party workers can call those who didn’t. Typically those people are volunteers. They must remain at a significant distance from the polling place, outside, and cannot hand out literature of any kind or otherwise campaign.