Downbeat Marker

The Downbeat Marker is a red line and is placed on the first transient of the track (this could be a kick drum, or first bass note at the beginning of the track). The Downbeat Marker will have a 1 above it to indicate it is Bar 1.

If the BPM is accurate, then the Beatgrid will be set perfectly and you will not need an further adjustment.
 
If the Beatgrid markers don’t fall on the track’s beats, then you will need to Edit the Beatgrid.