OK, so i'm going to do my best to summarise matched betting for you - it is essentially the process of placing bets in order to profit from promotions from betting companies (which means signing up to a bunch of betting sites to claim new joiner offers!). To make money through matched betting you must place bets that cover all outcomes of the bet.
This isn't the easiest side hustle to navigate, and I am in no way an expert! So if you want to know more, I recommend reading this article which covers all the basics.
Ps. it's legal, I checked!