A leech is a kind of worm that feeds by sucking blood from other animals. You might find yourself needing to pull a leech off your leg after swimming in a lake.
Because leeches are parasites, it's common for people to use the word as a metaphor for someone who sponges off of other people. If you have a friend who is always borrowing money but never pays it back, and who is known for requesting favors without granting them in return, you can call him a leech. The blood-sucking leech was once commonly used by doctors who treated patients by "bleeding" them, a practice that's still occasionally used in medicine today.