No doubt there are some shady characters trying to monetise “training courses” etc. in matched betting, which should be avoided like the plague, but the concept is sound and above board. There used to be a section on it on MoneySavngExpert, which if still there will explain it far better than I can.
I’ve done it a long time ago and made a decent profit out of it (around £1500). Its quite hard to make that much these days IMO as bookies are much more switched onto betting patterns associated to it and quickly throttle down your maximum bet to make it impossible to continue.
The simplest way I can describe it is that it is essentially a method for extracting money from the bookies who offer “free bets”. So for example if there’s a £50 free bet on the winner of the Grand National, you bet on horse X with that bookie, and lay horse X elsewhere (I.e. also bet against it winning). In that way you’ve covered all outcomes so will be able to get ~90% of the “free bet” back in your winnings (the other 10% being the spread/bookie’s margin) and cash out.
Not something to do IMO unless you are good at quickly doing Big Maths as one slip could mean that you’ve lost a lot.
Edited to add: Also stay away if you enjoy the gambling/excitement side of betting. If you move away from the absolutely clinical mathematical method that is required then you will be guaranteed to lose, as normal gambling is a total mug’s game.