Similar to a loop (a loop is often just a digitized groovelet), it's a... rhythmic feel, for lack of a better phrase. You have reggae grooves, ska grooves, soul grooves, jazz grooves... Origins come from rhythm and blues/black music, but there's also grindcore grooves, grunge grooves, etc. A groove can really only be played by live humans, as it gains most of its charm from minute human imperfections and interactions.