There are four kinds of strips: Action, Transition, Sound clip and Meta.
Action Strips are a container of keyframe data of an action. Any action used by the NLA first must be turned into an Action strip. This is done so by clicking the Push Down Action button see above. Alternatively, you can go to Add ‣ Action Strip.
Action Strips only playback the keyframe data that fits into the length of the strip. This includes any modifiers that might extend keyframe data.
Transitions interpolate between Actions. They must be placed in between other strips. Select two or more strips on the same track, and go to Add ‣ Transition.
Sound Clip Strips
Controls when a speaker plays a sound clip. Add ‣ Sound Clip.
Meta strips group strips together as a whole, so you can move them as one. If you find yourself moving a lot of strips together, you can group them into a Meta strip. A Meta strip can be moved and duplicated like a normal strip.
Add ‣ Add Meta-Strips
Group selected NLA-strips into a meta strip. A meta strip will group the selected NLA-strips of the same NLA-track.
Add ‣ Remove Meta-Strips
A Meta strip still contains the underlying strips. You can ungroup a Meta strip.