MIDI In Toggle ⌥I

Enable/disable step input recording from MIDI devices. When MIDI Input is “on” the MIDI In indicator turns red. The MIDI In and MIDI Out indicators are found at the top of the Event List, and at the top of the Piano Roll Editor

Overview of the Piano Roll Editor in Logic Pro — Apple Support

You can record notes into the Piano Roll Editor directly when the MIDI In button is activated using step recording techniques. See Use step input recording. Make sure the MIDI Out button is activated so that you can hear notes that you add, select, or click.

Use step input recording in Logic Pro — Apple Support

In Logic Pro, inserting notes with a MIDI keyboard is similar to using the Step Input keyboard. However, you press keys on your real-world MIDI keyboard, rather than clicking onscreen keyboard notes.