Matched Betting is a risk-free* way of making money out of bookies. Here's how it works:
Assume Bookie A offers a £10 free bet if you bet on x event.
Bookie B offers the same deal - there are dozens of bookies offering this type of deal and some are more generous than that.
So you place the bet with both bookies but with bookie A you bet on the home team to win and bookie B you bet on the home team not to win. This means you must win regardless of the outcome with one of the bets.*
However, you would probably lose a small amount of cash as the odds are always slightly in the bookies favour, let's say you got back £19 from the total stake of £20.
But this is the clever bit, you now bet on another match with the free bets. Again you will win and so assuming similar odds you get another £19 winnings, profit overall of £18.
It's a bit more complicated than that due to the actual variance of odds but that's the theory.
* Unless the game is postponed etc, then you would get your money back so you can try again.