Spread Betting is a form of betting derived from financial markets where the punter bets on a 'spread' of numbers relating to the particular event - for example: runs scored in cricket; points in rugby union; or the number of lengths between named horses at the end of the race. The bookmaker quotes the spread; say, regarding the number of points to be scored by one side in a rugby game, at 28-30. If you think the side will score more than 30, you 'buy' at 30; if fewer, you 'sell' at 28, and stipulate your stake per point. If you buy the spread at £1 and the team scores only 25, you lose £5 - 30 minus 25 leaves 5, which multiplied by your stake is £5; if that team scores 35, you win £5.
The attraction of spread betting is that the more right you are, the more you will win - and by the same token the more wrong you are, the more you will lose.
Note that you cannot have a spread bet unless you have an account with the bookmaker concerned, since you cannot stake the bet in advance as you do not know how much you might lose.