It's a pentangle, a five-pointed star. It's used in witchcraft. Lon Chaney, Jr., in Universal Studios, maintained that's the mark of the Wolf Man.
11 seconds sound clip from the An American Werewolf in London movie soundboard.
You can hear this line at 00:08:52 in the Blu-ray version of the movie.
Quote context
[...]
- Right. With Laurence Harvey. Everybody dies in it. Very bloody.
- Bloody awful, if you ask me.
- Here, Gladys, Tom. Did you hear the one about that rushing... Let me speak!
- Ask him what the candles are for.
- All right, then. I shan't tell it. No, no, you've had your chance.
- I see. You want me to ask them if they're burning candles to ward off monsters.
- Right.
- Wrong.
- So halfway over, the ocean the engines run low on petrol. So, they have to lighten the plane.
[...]