A siphon is a device used to move liquid from one container to the other. If you're giving your goldfish a magnificent new bowl, you can use a siphon to move the water from the old one to the new.
A siphon is a tube running from one container to another that uses gravity to cause liquid to flow. The key is that the container with the liquid has to be at a higher level than the container you’re moving the liquid to. You have to use force to start the initial flow, but after that gravity takes over. We also use siphon as a verb, for the process of emptying. An endless meeting might siphon off all your energy.