An abscess is a cavity full of pus that is very painful due to the pressure generated in the area. The abscesses in the sacrum and the perineum areas are usually caused by the fistulas. (Link to fistula section).
Treatment gives the pus a way out (drain the abscess). After this operation, pain practically disappears as the pressure caused by the pus is reduced. The treatment consists of a small surgical intervention, with local anaesthesia and no hospitalization needed. It is almost painless. There are usually some interventions after this operation that could produce some discomfort, but they are almost painless too.
In most cases, when the inflammation is gone, the fistulas that have caused the abscess should be treated.