What does apse mean?
The definition of apse
Apse, n.; pl. Apses. [See Apsis.]
1 (Arch.)

(a) A projecting part of a building, esp. of a church, having in the plan a polygonal or semicircular termination, and, most often, projecting from the east end. In early churches the Eastern apse was occupied by seats for the bishop and clergy. Hence:
(b) The bishop’s seat or throne, in ancient churches.

2 A reliquary, or case in which the relics of saints were kept.

Note: This word is also written apsis and absis.