Write routinely over extended time frames (time for research, reflection, and revision) and shorter time frames (a single sitting or a day or two) for a range of discipline-specific tasks, purposes, and audiences.

The only words that might be remotely confusing here are "range of discipline-specific tasks, purposes, and audiences," and even that's not too bad. It just means to give students a whole bunch of different writing assignments. And if students are following the other Common Core Standards, they should be doing that already. See, don't sweat it with this standard!