Shrink Space is to compact the table segment. This clause is valid only for segments in tablespaces with automatic segment management. By default, Oracle Database compacts the segment, adjusts the high water mark, and releases the recuperated space immediately.
Compacting the segment requires row movement. Therefore, you must enable row movement for the table you want to shrink before shrink space. Further, if your application has any rowid-based triggers, you should disable them before issuing this clause.