A piano roll is an editor that displays MIDI notes on a grid of time and pitch and lets you edit them. The horizontal axis is position in time and note length, and the vertical axis is pitch. Watching a keyboard-like scale, you can add, move, and resize notes to program melodies, chords, and drum patterns.
How it differs from a waveform editor
What a piano roll handles is performance instructions, not a recorded audio waveform itself. Even when you change the position or the length of a note, the sound is produced by a software instrument or an external MIDI instrument. That is a different target from editing recorded vocal or guitar waveforms and from pitch correction such as Flex Pitch.
Basic editing
Draw notes in empty space, drag to move the start position or the pitch, and pull the ends to change the length. Copy, duplicate, delete, and quantize let you shape rhythm patterns and chord progressions efficiently. Be careful with a grid that is too fine, since notes land in unintended places more easily.
Velocity and expression
Below the notes or in a separate lane, you can edit MIDI data such as velocity, modulation, and pedal. Velocity affects not only the level but also, depending on the instrument, the tone and the attack. Instead of making everything the same strength and length, vary it to suit the phrase.
Programming drums
In a drum instrument, kick, snare, hi-hat, and the rest are each assigned to a specific MIDI note. With supported instruments you can display drum names instead of a keyboard, so you can see which note is which sound as you place them. Check the mapping for each instrument.
Finish by ear
Even when it looks tidy on screen, the actual groove and articulation change with the instrument, the tempo, and the velocity. Always play it back after editing. How to adjust the length of many notes at once is explained in Tips for changing the length of MIDI data (Japanese).
