Ordinarily SLEEP_TASK is a nonspecific wait type in SQL Server which occurs when a task sleeps while waiting for a generic event to occur, according to Microsoft documentation. This wait type can usually be safely ignored, however on some occasions it can happen when a script does not execute completely or hangs up for long periods of time.
The SLEEP_TASK wait means that a thread is waiting on a resource or waiting for some event to occur, and could indicate background task scheduling, a query plan exchange operator that isn’t tracked by CXPACKET, or it could be a hashing operation that spills to tempdb.
Read the whole thing. For a bit more information, check out the SQLskills description of this wait type.