The mercenaries are private contractors that will fight for the highest bidder. As of now, the Mercenary faction isn’t a real army in the Human Sphere but you can use units within other armies. They are rumoured to be available as an fighting force on its own in the next edition.
Most of the units are professional soldiers like the Druze maffia and their TAG warrior Scarface, others are warriors fallen out of grace like Saito Togan, privateers like McMurrough, fugitives like Avicenna, or just plain old pirates like the Yuan Yuans. Others are more like the private army of some big company like the Kaplans who work for the Beyhan Resources. In the Tournament System, only some mercenaries are allowed in certain factions, so you can’t run an entire mercenary filled faction.