If you drain something, you remove the liquid from it, usually by pouring it away or allowing it to flow away, and if something drains, liquid flows away or out of it:

The meaning of DRAIN is to draw off (liquid) gradually or completely. How to use drain in a sentence.

To or into the condition of being wasted or lost: All of our best laid plans are down the drain.

DRAIN definition: to withdraw or draw off (a liquid) gradually; remove slowly or by degrees, as by filtration. See examples of drain used in a sentence.

Drain (plumbing), a fixture that provides an exit-point for waste water or for water that is to be re-circulated on the side of a road Floor drain Drain (surgery), a tube used to remove pus or other …

drain /dreɪn/ v. to empty by drawing off liquid: [~ + object] to drain a swamp. [~ + object (+ of + object)] Drain the wound (of blood) before you apply the bandage. [no object] The crankcase …

If you say that something is a drain on an organization's finances or resources, you mean that it costs the organization a large amount of money, and you do not consider that it is worth it.