A count-in gives you a bit of a head start before recording by engaging the SPL one or more bars early.
In the Song Settings window, select the Recording pane.
Make sure the Count-in setting (Logic Pro only) is checked.
From the Count-in menu, choose 1 Bar.
The Count-in menu will let you set up to six bars as a count-in.
Close the Song Settings window.
In the Arrange window make sure that an Audio Instrument track is selected.
At this point, you're just going to test the count-in. MIDI-based tracks do not record anything unless you generate MIDI data. You don't need to record anything just yet, so for the purpose at hand, an Audio Instrument track makes a good test subject.
Position the SPL at bar 5.
Make sure Cycle mode is turned on.
If Cycle mode is on, Logic begins recording at the beginning of the cycle mode instead of at the SPL's position.
Click the Record button (press * on the numeric keypad).
The Bar Ruler turns red (for record), the SPL jumps back to bar 4, and it then starts playing. This one-bar jump-back represents the 1 Bar count-in you selected in step 3. During this one-bar count-in, nothing will be recorded, but the metronome clicks along to get you into the tempo and prepare you to record.