A Licensee is any third party you license a track to, typically a record label. They will usually include your track(s) on a compilation or another release and will account back to you royalties from their sales.
The amount they account back to you depends on the agreement that you signed with them.
Each time you license a track to a third-party Licensee you should create a "Contract Out" in RoyaltyWorx to record all of the deal information.
The following articles will provide more information.