Label is an element of Timeline used to mark key frames. Labeling frames has no impact on how they work, this is done for visual convenience.
A key frame (at the center) marked by a label (red line)
A label is represented on the Timeline as a red line marking the labeled key frame. Only key frames can be labeled.
To add a lable toa key frame, select the frame and click the red flag on the upper part of the timeline:
Clicking this button will add a label to the selected key frame
To remove a lable from the key frame, select the frame and click the red flag one again.