Midi lengths refer to the duration of MIDI (Musical Instrument Digital Interface) notes, which dictate how long a note is played in a digital music composition. Each MIDI note has a specific length measured in ticks, which are subdivisions of a beat. This allows for precise control over timing and rhythm in music production. In MIDI sequencing, adjusting the lengths of notes can create different musical effects, such as staccato (short notes) or legato (smoothly connected notes). Musicians and producers often manipulate midi lengths to enhance the expressiveness of their compositions, making it a crucial aspect of digital music creation.