There are two types of Markers,
1. Clip Markers
2. Timeline markers
To add a clip marker select the clip & while its playing or place play head on a particular frame and hit M this will add a clip marker. Similarly to add markers on the Timeline you hit M and the marker is added to Sequence in the timeline.
Shift + M to go to next marker
Option + M to go to previous marker.
To edit marker place the play head on the marker on the timeline and hit M, and you can rename or add comment to the marker
Chapter Markers <chapter> added for DVD menus to jump to particular location.
Compression Markers <compression> you can add this when there is rapid movement and you get some compression problem. let’s say MPEG video !
Scoring Marker This is needed with SoundTrack Pro, to specify where a piece of music is suppose to start or where it is supposed to end