Dephlegmators are a pre-condenser, typically at the top of the column, which sends distillate back towards the pot.
Acting as a cooler that distillers can control with precision, dephlegmators help increase the reflux in the column, and allows them to better control the rate at which the spirit can travel through to the condenser. This greatly aids the overall purity of the spirit, as well as being a useful tool to help increase the ABV of the spirit before collection, and therefore can also help increase yields when it comes to certain spirits such as Gin.
Some people refer to their reflux condensers (namely American distillers as they are more common there) as dephlegmators, but while similar, they are not quite the same. Dephlegmators use warmer/hot water (many hybrid stills use hot condenser water to feed their dephlegmators) rather than a separate cold-water feed. This is in contrast to a typical reflux condenser which does indeed control the reflux ratio but is not as finely tuned.