Deceptively simple yet useful to boot. Bring the playhead anywhere on the timeline, smash the i key and BAM! the mark in is set. Ok but what now? well, move the playhead down a bit and slam the O key. Mark out is set. As of writing, I use these keys in the Deliver [Shift + 8] Panel to render only a portion of the timeline.
In/Out Range made easy with i and o keys