Seal doors is a monster-only spell which seals shut not only doors but stairs as well, making them impassible/unusable until the duration of the effect ends, the vault warden which created the seal is killed, or the seal has been destroyed. A door that is sealed shut will immediately shove any items, creatures, or even you out of the way in the process of closing; there is no way to prevent the effect.
The following monsters can cast seal doors:
Destroying a sealed door
The following can be utilized as a means of destroying a sealed door:
However, note that sealed staircases cannot be destroyed (what would you gain from this anyway?) and the only way to lift a seal is to wait for it to dissipate (not advisable since the vault guard will most likely re-cast it immediately) or to kill the vault guard.
Seal doors was added in 0.12 along with the vault wardens.