A bull run is a sustained period of strong performance. It may be applied to a market, a sector, or a single security. During a bull run, bulls dominate the market for an extended period.
If a bull run becomes too strong, it may turn into a bubble, which then leads to a crash. Otherwise, it may be moderated by corrections, or simply taper off before valuations become too high.
Momentum investors may make money by joining in bull runs, but it can be difficult to judge when the bull run will end, very likely in a sudden movement downwards.