A MIDI file contains no music. It records how music is played, containing “events” which control when each note of each instrument starts, finishes, how loud, what note of the scale. It has to be played by the sound card’ synthesiser. It will sound like the particular instrument that is playing it.
A .WAV file contains a digitally sampled recording of an actual sound, be it music, speech whatever. It is played by the DAC of a sound card, in the same way that a CD player plays the data from a CD. It will always sound like the original sound that was recorded.
A MIDI file is very compact, a .WAV file will be very large in comparison.
Before loading MIDI file into the Wave Editor you need to render MIDI into waveform using MIDI to WAV converter like MIDIRenderer. After conversion you will have standard WAV file that can be loaded in Wave Editor.