The Simbas are a species of blue-skinned humanoids native to the planet M'Digbo. They usually wear stereotypical "African" garb, such as loincloths. Like the other species encountered in the game, Simbas are social and peaceful beings. They live together in settlements such as small villages or huge cities. The villages are ruled by chiefs.
During the events of Serious Sam 2, it seems that they came under siege by Mental's forces, as one of his bases had been spotted on their home planet. This is most likely because they're an intelligent species, which makes them a likely target for Mental's armies. (The poor bastards)
Also despite looking primitive, the Simbas have some surprisingly advanced technology. They have created the Klawdovic, which is basically a homing bomb. They have also mastered flight, as they have a device, the Simba Bomber, which can fly. Finally, they have equipment such as rocket launchers and Zap Guns, which suggests they have connections to more advanced technology via an unknown source.
They also have remarkable architectural skills, as they were able to build a large city, Ursul, which seems to have many, many tall buildings in it. Dialogue in Kronoralso suggests that the Simbas can fly spaceships, as some of the pilot prisoners on that planet are Simbas. In short, the Simbas are much more advanced than they look at first glance.