MIDI stands for Musical Instrument Digital Interface, and is a polyphonic digital control protocol. It can be sent between hosts and devices like keyboards, drum machines, sequencers and Digital Audio Workstations (DAWs) over 5-pin MIDI cables, or USB. A MIDI signal is able to transmit information regarding when a key is pressed and released, which key or keys are pressed, how hard they are pressed, and other things like the use of a sustain pedal, pitch bend and modulation wheels, and a number of other controls which are referred to as continuous control (CC) messages. Because MIDI is control data, it doesn’t sound like anything on its own, but it can be used to control a wide range of software and hardware instruments.
Midi can be converted to CV in order to control a Eurorack synthesizer, but CV can also come from a number of other sources.