Are you using columnstore indexes?

Documentation for that wait type is basically nonexistent (Paul Randal's compendium has it, but says nothing other than that it was added in 2016).

Since it was added in 2016, my best guess is that it's related to the new behavior for row group merging and maintenance for columnstore indexes in 2016 (see https://blogs.msdn.microsoft.com/sqlcat/2015/08/17/sql-2016-columnstore-row-group-merge-policy-and-index-maintenance-improvements/ and http://www.nikoport.com/2015/12/25/columnstore-indexes-part-74-row-groups-merging-cleanup-sql-server-2016-edition/).

Not sure whether it's benign or not, but to speculate further, I'd guess it is. At any rate, it's all wild speculation from me at this point, since I don't have access to a 2016 server for testing at the moment.

Cheers!