“Metre” is the British spelling of the unit of length equal to 100 cm, and “meter” is the American spelling of the same unit. However, “meter” is also used in British English, but it means something different.
A “meter” in British English is an instrument for measuring. You have several of them at home – a water meter, a gas meter and an electricity meter. The man with a clipboard who rings your doorbell when you’re just about to step into the shower on a Saturday morning has come to read your meter. And you put money into them when you park on the side of the road in the city centre – parking meters. “Meter” also has this meaning in American English.