The trading window is the window of time each weekday when M1 makes all trades for user accounts. M1's trading window benefits users because it helps keep M1's management fees low since M1 is only trading one time per day. M1 is a long term investing vehicle, not a trading platform, so timing of trades is less important.
M1's trading window occurs at 9am CT everyday the NYSE market is open. All changes to your portfolio made before 9am CT on days that the NYSE is open are generally executed the same day during M1's trading window.