A boom-bust player in fantasy terms is someone who could perform well, but could also perform poorly. Players like this are a high risk high reward selection that could make or break a fantasy football season.
Drafting a boom-bust player obviously comes with a risk. It can result in a player being one of the best players on your team as well as dropped only a couple weeks into the season. It can depend on the situation, but in most cases drafting such players is usually not a very good strategy. These types of players' performances cannot be predicted and as such you put your season's success out of your control. Drafting these players can be compared to gambling.
David Johnson was an early first round pick in most 2018 fantasy drafts, and owners that drafted him expected a surefire fantasy stud. What he produced was equivalent to a mid tier running back. He had a high upside after leading fantasy leagues in 2016 but turned out to be a bust.
However, as discussed, for every positive outcome there is an equal chance at a negative one. Drafting a player that is expected to be very good and who turns out as a bust severely hurts a team. Lack of production from someone who should be a star player is sometimes too much to overcome and can be enough to sink a team's season.