The Piano Roll Editor shows notes in MIDI regions as rectangles on a time grid. The position of each note shows the time position where it starts playing, its duration (length), and pitch.
You can edit individual notes by moving them, resizing them, dragging them vertically to change their pitch, and in a variety of other ways.
Piano Roll Editor menu bar: Contains the MIDI Draw button, Catch button, and zoom slider.
Piano Roll Editor inspector: Contains controls for quantizing note timing and adjusting note pitch and velocity.
Display area: Shows the notes in the MIDI region or regions as bars on a time grid.
Ruler: Shows time divisions so you can align and edit notes at precise time positions.
Playhead: Shows the current playback position.
Do one of the following:
Double-click a MIDI region in the Tracks area.
Select a software instrument track, then click the Editors button 
in the control bar.
Select a software instrument track, then choose View > Show Editors.
For more information about using the Piano Roll Editor, see Piano Roll Editor overview.