The blue cursor, in the detailed and preview waveforms, is the cue point. By default, it is set at the beginning of a track, but can be moved anywhere you need. By using the cue play buttons you can:

Cue points are saved between sessions. This means that if you set a cue point in a track in your library, the next time you load that track, the cue point will already be there.

Add Cue Point

You can add up to 8 cue points for each track. When you have used up all 8 cue points you will need to overwrite an existing one.

Strict Mode

This option automatically locks the cue point position to the position of the nearest beat marker. This allows you to easily align multiple track beats while playing. This option will also preserve the tempo while playing a track and Cue Play is used. This can be done by the Deck Menu or by using the hotkey.