(covers information from several alternate timelines)
Swarm drones were a drone workforce believed to have been left behind on Altamid by the planet's former inhabitants sometime prior to the mid-22nd century. They were repurposed to serve Krall in the following century.
Following the crash of the USS Franklin on Altamid in 2164, Starfleet Captain Balthazar Edison discovered technology abandoned by the planet's previous inhabitants – among them advanced mining equipment and a sizable drone workforce. As he transformed into the despot known as Krall, Edison adopted the drones to serve as his personal military force, using them and their ships to capture other alien vessels and imprison them on Altamid.
In 2263 of the alternate reality, Krall and his fellow Franklin survivor Manas attacked the USS Enterprise, using these drones to board the ship and kill many members of its crew, though some of them were cut down in return by the Enterprise's security forces. As she attempted to help Captain James T. Kirk save his ship, Lieutenant Nyota Uhura engaged in hand-to-hand combat with drone soldiers.
Following the destruction of the Enterprise and the capture of much of its crew, Krall and Manas used the drone force to corral dozens of Starfleet crew – including Uhura and Lieutenant Hikaru Sulu – into prison cells at Krall's base of operations. During this time, Krall's other cohort Kalara was accompanied by several drone soldiers to retrieve the super weapon Abronath from the remains of Enterprise, crashed on the surface of Altamid.
Krall later led his force of drones to the Federation Starbase Yorktown where they attacked the outpost, but were defeated by the former crew of the Enterprise, who used a specifically calibrated radio broadcast from the Franklin to disrupt the swarm's communications and ultimately destroy them. (Star Trek Beyond)
Resembling insect-like humanoids, these drones were equipped with arm-mounted particle weapons and were heavily armored. They were capable of piloting small spacecraft to perform complicated attack maneuvers as demonstrated in their attack on the USS Enterprise.
Despite their skill in warfare, these drones were incapable of working independently, relying on networked cyberpathic communication. (Star Trek Beyond)
Krall's Swarm drones are credited to several actors listed as "Swarm Soldier" in the credits for Star Trek Beyond. The drones were digital creations, using stunt men as stand-ins before being rendered in CGI and placed into the finished film. 
Multiple concept sketches of "Swarm soldier"s can be found at Trekcore.   Early computer-generated versions of the Swarm drones can be seen in the film's first trailer, which was released on 14 December 2015. 
Discussing the Swarm drones and their relationship to Krall, Beyond co-writer Doug Jung stated, "Justin's idea was that [the soldiers] were sort of like drones in a way, and that they don't actually have a lot [of] conscious thought of their own. That sort of answers how Krall would be able to come in and take all this stuff [....] They weren't a society that had weaponized anything."