Geordie (Northeastern British) slang. It means OK, correct, or righteous. Or more commonly, just a mispronunciation of "right".

For example, "Yar aal reet, marrow." means "You're all right, friend."

It also shows up in Harlem slang ("zoot suit with a reet pleat") and in jazz/swing music, an example being the Jackie Wilson song, "Reet Petite".

And it works quite onomatopoetically for the violin music used in the shower scene from Psycho. Reet! Reet! Reet! Reet!