This is a bet on when the first try of a game will be scored, measured in minutes.
If you were to bet low on the time of the first match try then ideally you would like a first minute score. However if there were no tries in the match then the make-up would be 80 (as in 80 minutes). SportingIndex offer a market for the first match try (scored by either team) and separately for both individual teams.
For example, for a Six Nations match between Scotland and Italy, SportingIndex predicted the first match try (by either side) would be scored after 18-21 minutes. If you were convinced the match would be deadlocked with no try for far longer than this you might bet high at 21 for say £5 per minute.
If the Scots only managed to break through the Italian defence to cross the line in the 27th minute then you would have won 6 times your stake: (27 - 21) x your stake = 6 x £5 = £30. However, if you had gone low at 18 minutes for the same stake, you would have lost 9 times your stake: (18 - 27) x your stake = -9 x £5 = -£45.