„to trigger” means to cause or to initiate. For me it means that I trigger a display, an order proposal or a direct order, if a toner has reached the percentage value specified by you. If, for example, you have entered “5%” as the trigger level, I will trigger the order behaviour you specified (Monitor = display in overview, Suggestions = order proposal + mail, Automatic = automatic order at Topcart) for the cartridges of your printer if I notice that only 5% toner is left.
By the way: You can also totally individualise the trigger value! Not only generally for black, cyan, magenta and yellow, but also for every single printer. This can be done via the individual device settings.