From BlenderWiki

Markers

Markers are used to denote frames at which something significant happens – it could be that a character’s animation starts, the camera changes position, or a door opens, for example. Markers can be given names to make them more meaningful at a quick glance. They are available in many Blender’s windows, under different forms. Unlike the keyframes, markers are always placed at a whole frame number, you cannot e.g. set a marker at “frame 2.5”.

When you create a marker in one of the windows, it will also appear in all others that support them:

• The 3D View window.
• The Ipo Curve Editor window.
• The Action Editor window.
• The NLA Editor window.
• The Video Sequence Editor window.
• The Timeline window.
• The Audio window.

Pose markers

There is another type of markers, called “pose markers”, which are specific to the armatures and the Action Editor window. They are related to the pose libraries, and are discussed in detail here.

Visualization

Standard

Most of the window types visualize markers the same way: as small triangles at their bottom, white if unselected or yellow if selected.

If they have a name, this is shown to their right, in white when the marker is selected. See (Markers: small but useful).

Sequencer

The Video Sequence Editor just adds a vertical dashed line to each marker (gray if the marker is unselected, or white if it's selected).

3D View

The 3D View windows do not allow you to create/edit/remove markers, they just show their name between <> at there bottom left corner, near the active object’s name, when you are at their frame (see Marker in a 3D view).

Editing Markers

Unfortunately, there is no common shortcuts and menu for marker’s editing, across the different window types that supports them… So in the refboxes of each action described below, I put the most-common shortcut and menu entry, with the known exceptions between brackets.

Creating Markers

The simplest way to add a marker is to move to the frame where you would like it to appear, and press M (or CtrlAltM in a video sequence editor).

Alternatively, you can press AltA (or the “playback” button of the Timeline window) to make the animation play, and then hit M (or CtrlAltM in VSE) at the appropriate points. This can be especially useful to mark the beats in some music.

Selecting Markers

Click RMB on the marker’s triangle to select it. Use ⇧ ShiftRMB to (de)select multiple markers.

In the Ipo Curve Editor, Action Editor, NLA Editor and Video Sequence Editor windows, you can also (de)select all markers with CtrlA, and border-select them with CtrlB (as usual, LMB to select, RMB to deselect). The corresponding options are found in the Select menu of these windows.

In the Timeline and Audio windows, you can (de)select all markers with A, and border (de)select them with B…

Naming Markers

Having dozens of markers scattered throughout your scene’s time won’t help you much unless you know what they stand for. You can name a marker by selecting it, pressing CtrlM, typing the name, and pressing the OK button.

Moving Markers

Once you have one or more markers selected, hit CtrlG (or G in Timeline or Audio windows) to move them, and confirm the move with LMB or ↵ Enter (as usual, cancel the move with RMB , or Esc).

By default, you grab the markers in one-frame steps, but if you hold Ctrl, the markers will move in steps corresponding to one second – so if you have set your scene to 25 fps, the markers will move in twenty-five-frames steps.

Duplicating Markers

You can duplicate the selected markers by hitting Ctrl⇧ ShiftD (or ⇧ ShiftD in a Timeline or Audio window). Once duplicated, the new ones are automatically placed in grab mode, so you can move them where (or rather when) you want.

Note that unlike most other duplications in Blender, the names of the duplicated markers are not altered at all (no “.001” numeric counter append…).

Deleting Markers

To delete the selected marker(s) simply press ⇧ ShiftX (or X in a Timeline or Audio window), and confirm the pop-up message with LMB .