A player who is range merging will bet marginal hands as they have a good read on their opponent and expect they will call with a bluff catcher.
In other words, the range of hands bet changes from being polarized (very strong and weak hands) to betting a merged range which contains lots more value hands that can beat bluffcatchers.
This can be a profitable strategy against players who payoff too much. When playing against such opponents, it would make little sense betting your weaker hands (bluffs) on the river.
Here is a great example of a player who was “range merging” at the EPT10 London Main Event.