they mean midi output from resolume so that parameters controlled via hardware or external sequencers can update their position on the hardware when changed in resolume.
The easiest example of this to understand is a controller such as the korg pad kontrol - it has 16 buttons which can light up. Lets say we want to assign them to 16 clips in resolume. It would be very nice if the lights in the buttons indicated which clips were currently playing. To do this we need resolume to send midi out which shows us when the clip is playing (or more specifically when it stops playing) Then the button light is easy to control - the NoteOn command which triggers the clip is bounced back to the controller which uses it to light up the buttons LED, then when the clip reaches the end and stops it sends a NoteOff command to the controller which turns off its LED.
This functionality has been in audio software forever and its badly missing from VJ apps. I would very much like to see this kind of function in resolume. (Hardware manufacturers will also like this - they will even send the resolume team free controllers to test with