An edge in sports betting is the ability to find bets that have a higher probability of happening than the odds you receive.
Since this is the best way at becoming a successful sports bettor, it's not easy to find an edge. Bookmakers set odds with the help of data, often outmatching anything a single individual can produce. If you can find an edge on a certain side, the next part is finding the best odds at the available sportsbooks.
If you want to be successful, this is something that takes time and may not be attainable if you don't have the tools. You can find edges a number of ways, whether in injuries that get reported before sportsbooks can update their odds or even a developing weather pattern. Either that, or you find something in a head coach's press conference, talking about how a certain player will get more or less playing time.
If you aren't a mathematician, these are small things that give you edges over the sportsbooks. Of course, finding news before sportsbooks can get to them is rare. Prior to the internet, it was easy to find a local news nugget before a sportsbook, but that's no longer the case.
There are other methods to finding an edge, namely if you have a system that ranks a given set of teams more accurately. For example, you can create your own system of setting betting lines (power rankings) and it turns out to be better than whatever the sportsbook produces. Again, this is something that takes time.
Finding an edge over the sportsbooks isn't easy and that's why becoming a professional sports bettor is easier said than done. While finding edges are possible, they aren't something that pop up every day.