Its a musical term for playing a note, possibly with an initial downward slide for a semitone or so before glissando-ing up indefinitely.
The initial Doi is equivalent to how you' say Doi as in "you're retarded" and then add a highly pitched "it".
Saying "do" low then "it" in a high pitched voice is close but not correct. it has to go from the pitched note down a bit first, then glissando-ing up to infinity.
A Word commonly used by film teachers directed towards there students, It often ends a long schpeel and tells the students that the teacher is done and they may now begin there work.