A "floating window" is a range of dates you specify, that floats. You specify a period of days from today for it to start, and a period of days from today for it to end. For example, you can make a window that starts 14 days from today and ends 45 days from today. That "window" will update, or float, every day, so that it always refers to the period of time that starts 14 days from the current day, and ends 45 days from the current day. You can make "windows" of any size and make pricing rules for them.
A "floating window pricing rule" assigns a pricing rule to a window you create. A common floating window pricing rule is to decrease prices gradually over the next 7 days by 40%. This means that the floating window you create will start 0 days from now and end 7 days from now. The rule will be to gradually decrease prices by 40%. So that night's price will be 40% lower than your base price, the following night will be 35% less, etc. This is done to try and attract last-minute bookings so your STR does not sit empty, and make you lose out on profits.