Imagine you are in the fourth grade (or any grade for that matter). Your teacher launches a new science unit (or any content for that matter) and puts the following directions on the board:
Quick Write: What do you already know about energy?
(or another topic for that matter)
What do you do? In other words, what does it mean to quick write? As with all WTL activities:

"WTLs give us starting points for new units: we have kids write to surface their prior knowledge, including misconceptions, to activate their thinking, to set class and individual goals for an upcoming unit. Along the way, we use WTLs to help students stop and collect their thoughts, sort out ideas, notice and hold their thinking, review and readjust goals, and get ready to move ahead. Later in a unit, we use different kinds of short writing to help kids synthesize what they have learned, connect with others, compare notes or plan projects or outlets for their learning."
--Content-Area Writing
Check out the discussion of quick writes (and Tom Hanks) on WTL's sister blog Try-It Mentor.