Markers and in and out points

Press M to add a marker at the playhead, or ⌥⇧⌘M while you record. Press I and O to mark the start and end of a stretch, then cut it with X or export just that part.

Markers

Markers are small flags above the timeline.

While recording, press ⌥⇧⌘M or click the flag on the recording bar. Use it for a moment you want to find later, like a mistake.
In the editor, press M to add a marker at the playhead.

Click a marker to jump to it. Right-click it to remove it. Markers you added while recording say recorded when you point at them.

Markers also set the chapters of a launch pack.

In and out points

Move the playhead to the start of a stretch and press I.
Move to its end and press O.

The timeline shows the range and its length. Now you can:

Press X to cut the whole stretch and close the gap.
Export only that stretch: in the export window, set Range to the marked times instead of Whole video.

Press ⌥X to clear the marks.