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.
Shortcuts:
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
Special Markers
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