Mido is a Python library used to interact with MIDI data, allowing developers to read, write, and play back MIDI files. It provides an object called MidiFile that can be used to open, modify, and save MIDI files. The library also allows developers to loop through the tracks in a MIDI file and examine their properties, including the number of messages and type. By using Mido, developers can clean up duplicate tracks in MIDI files, mash up different versions of the same song, or use it as an interface for digital instruments.