At my first open the arbiter spent most of round three sitting on a stool at the front, doing nothing visible. That is the job done properly. You notice an arbiter when something has gone wrong, which in a hall of eighty boards is maybe four times a round.
Before the round starts
The pairings come out of the software and go on the wall, usually fifteen or twenty minutes before the start. Boards, sets and scoresheets are laid out, the clocks are set to the announced control and tested, and the arbiter decides which side of the board each clock sits on. At the announced minute, White’s clock is started, whether White is in the chair or in the car park. How long a latecomer has before they forfeit is set in the event regulations, so read them.
During the round they watch, and they never coach
Article 12.1 gives the whole job in a line: the arbiter shall see that the Laws of Chess are observed. Article 12.2 asks them to act in the best interests of the competition.
Neither of those includes helping you. An arbiter will not tell you it is your move, will not remind you to press your clock, and will not mention the rook you have left hanging on b2. They will look at the ceiling.
Illegal moves are the exception, and the treatment depends on the speed. In a classical game an arbiter who sees one steps in. In rapid and blitz they generally leave it to the players to claim, which the appendices to the Laws set out and the briefing before round one will confirm.
Claims are most of the work
The list at an ordinary weekend event is short and repeats itself: a flag fall, a threefold repetition (Art. 9.2), the fifty-move rule (Art. 9.3), an illegal move, a touch-move argument, a phone going off in someone’s bag.
They all get the same procedure and they all need evidence, which is your scoresheet. A repetition or a fifty-move claim is checked from the written moves, and a player who has stopped writing has nothing to show, which is one more reason to keep the scoresheet up to date.
The penalties come from a ladder in Art. 12.9 and it runs from mild to fatal: a warning, then time added to the opponent, then time taken off the offender, then the game declared lost, then points deducted, then expulsion from the event. The rung you are most likely to meet is two minutes to your opponent for a first illegal move. Phones are the fast route to the sharp end: under Art. 11.3.2 a player found with a device on them in the playing venue can lose the game on the spot.
Who they are
Arbiters are licensed and examined. The ladder runs national arbiter, FIDE Arbiter (FA), International Arbiter (IA), and the person on the stool has read section E of the Handbook far more carefully than you ever will.
So use them early. The mistake I see beginners make is trying to sort out a dispute politely with their opponent for three moves and then calling the arbiter once the position has changed and nobody can reconstruct anything. Hand up first, conversation second.