"in a/one corner" refers to the place where two walls meet.
In a room with 4 walls, there will be 4 corners.
In a boxing or marshal arts ring (ring is round but in this case the ring is square) there are 4 corners.
When children or students do not behave properly, the parent or the teacher may tell the child to stand in the corner (facing the corner). This is a form of punishment.