When Placing A Bet On The Football There Is A Bet Called Round Robin. What Does This Bet Consist Of?

It's not actually a single bet, but a group of combined bets, called "parlays" in the lingo.

A parlay is group of bets on event (game)  outcomes.  All of the individual games have to turn out as you predict in the parlay for you to win at all.  

A Round Robin, in turn, is a group of parlays. All of the games in each of the parlays have to turn out as you predict for you to win anything at all.

Obviously the odds of every game in every group turning out as you predict are very small.  Which means the bookie will give long odds, which means that you can put down a relatively small amount of money and get huge winnings back (although the odds are always that your money should be considered as kissed goodbye).

Different bookies offer different Round Robin deals,  but usually they involve just 2 to 4 teams in each parlay, and no more than 3 or 4 parlays in the entire Round Robin.

I am pretty sure that all such gambling is technically illegal in the USA, though the authorities struggle to stop it.

In Britain a round robin is three doubles three singles and a treble. That is seven bets. Singles are bets on each horse etc individually to win. Doubles mean you must have at least two to win. Were three horses are picked that offers a combination of three possible doubles. i.e horses a b and c would be worked out as follows. a+b, b+c, and finally a+c. The treble is only paid when all three win.