Roughly "filled up". As your nutrient solution is taken up by your plants and evaporated, the reservoir need to be "topped off" to maintain the same volume of liquid.
Same as in a coffee shop when you've drunk about half the cup and the waitress comes by with a pot and tops it off for you...
yes. Topping is a method of cutting off the apical shoot in order to redirect hormones to produce more flowering sites. Not all strains respond well to topping, while others thrive when topped. There are various forms of topping too; actually cutting the top off, pinching the top 2/3 of the apical meristem (known as the FIM method), pinching the stem instead of cutting it.... There are a bunch different methods. I need to go Top Off my gas tank, and then go top off some plants. I would hate to have to learn english as a second language. Props to all the polyglots.