It is a slang and means :
a. to steal or pilfer.
b. to rob or steal from.
c. to swindle, cheat, or exploit; take advantage of: phony charity appeals that rip off a gullible public.
If someone rips you off, they cheat you by selling you cheap stuff not worth the money they are charging or by getting more money out of you. They can also steal your ideas, work and claim it as their own.