It means you've got to drain the fuel into the engine. They do that themselves these days.

In my day we had to drain fuel manually from the tanks before the first flight of the day. Condensation forms overnight, especially in a tank with little fuel in it. This sinks to a small sump at the bottom of the fuel tank & can be drained into a glass container by holding the drain valve open with a thumbnail. If you're not careful the fuel runs down your arm & under your armpit. Very painful a few minutes later I can tell you. I used to run my old motorbike on the fuel I collected draining tanks during the week.

PS. This brings back fond memories of balancing precariously with one foot on the main wheel of a Tiger Moth, a milk bottle in one hand & operating the drain valve with the other. I had to do this on each of our 3 aircraft every working day for something like 2 years & sign for doing so.

It depends where the tank(s) is located. The Tiger Moth was purely gravity fed as the tank is high up on the centre section. On an aircraft like the Piper Cub with the main tank just behind the engine there would most likely be a manually operated priming lever on or near the fuel pump. You would need to establish a flow to the carb before attempting start the engine.