Mercenaries were groups of people that pretty much had no loyalty to a single nation. The fought along with nations that paid them (roman empire in this case).
A mercenary is someone hired to temporarily serve in a foreign army, they are normally specialized fighters with a specific skill, for example, archery. Unlike an official soldier of an army, a mercenary doesn't serve for political interests but for money or any other personal interest.
In the Roman Empire, mercenaries were non-Roman citizens hired to fortify the Empire's borders from any external attack or offensive. If they served for 25 years, they were given citizenship and rewarded with a plot of land to farm in their retirement.
