Thralls are slaves, but are generally well-treated and have the prospect of eventual freedom. The children of thralls are free. You can become a thrall if
– you are captured in battle and nobody pays your hostage-price
– as punishment for certain serious crimes
– you are captured on a viking raid in foreign lands
– you incur a large enough debt to someone that you can't pay
Debt- and punishment-based thralldom generally has a fixed term, not "life". Because your thralls might be your neighbors' relatives, and will someday be free members of the community, people generally don't treat them too badly.
Thralls are expected to fight in defense of the homestead but not expected to fight otherwise, i.e. in raiding parties. The exact relationship between a thrall and his masters varies a great deal from case to case. A thrall who volunteers to help on raids will be more respected and may well be freed early.
If any players want to be thralls, they will be assumed to have a pretty good relationship with the Skeggvasons.