A henchman is a follower who will join a PC as an associate and fight by their side. Some modules provide henchmen as part of the story being told, while others provide them merely as helpers for the PC, and still others do not make henchmen available. (See associate for the pros and cons of using henchman help, including the party size penalty.)
The number of henchmen that can be used at one time is set by each module's builder. In the original and Shadows of Undrentide campaigns, PCs are limited to one henchman at a time; in the Hordes of the Underdark campaign, they are limited to two.