A contraction of the words "ja" (yes) and "nein" (no). It means that you agree to what has being said, but that you disagree about one aspect that has been mentioned or that you restrict your affirmation.
"Kann ich mir dein Auto ausleihen?"
"Jein, höchstens für die nächste halbe Stunde. Dann brauche ich es selber."
"Can I borrow your car?"
"Yes, but only for the next half hour. Then I need it myself."