The extra rowtime bounds that a data source adds as it inserts rows. These give advance notice of future data: namely, a lower bound on the rowtime of all future rows inserted. Asserting the bound t means that, for the next inserted row r, the field r.rowtime will be at least t. Other ways to express this are that the “clock” of the stream of inserted rows has jumped forward to t, or that the data source is going to be “quiet” until time t.