Chapter serfs are the normal human bondsmen and servants of a Space Marine Chapter. The Space Marines themselves are too few in number to manage all the tasks required to maintain all the aspects of the Chapter, its fleet, fortress, and other myriad aspects. It is the serfs who perform most of these vital functions.
A Chapter's fortress-monastery is home to a large population of these hereditary servants. Although they occupy a humble position, they are loyal members of the Chapter. They are generally descended from individuals selected as potential recruits to the Chapter, but in the end, judged unfit to become actual Space Marines.
The serfs are still inducted into the Chapter cult, and do not begrudge the status they could have obtained. They are generally well treated within the Chapter, are educated and trained to a much higher standard than most other servants of the Imperium, and have access to a better range of equipment.
When Space Marines go to war, the serfs maintain the fortress and defend it from attack. It is unlikely that the Chapter Serfs would ever be deployed in battle alongside their Space Marine masters, as their human limitations would be incapable of keeping pace. They would still be trained to levels at least equivalent to that of the Imperial Guard, expected to assist in the defense of chapter assets such as fortresses and vessels. Indeed when facing down a Space Marine Battle Barge it would be a chapter serf manning the gun turrets, as a full Space Marine would be too valuable for the task.