The tracker node is a powerful tool which will allow you to track a group of pixels as they move across a plate. The track can then be used with a transform node and merge node to layer something on top of the tracked plate to place something into a scene and have it move with it
The list of tracks is empty by default, so you will need to click the add track button to add a track. Once you have added the track, it should have appeared in the viewer.
By clicking and dragging, you can place the crosshair onto the middle of the pixels, use the inner box to select the group of pixels, and use the larger box to tell nuke the area to search for those pixels in. Once you have placed the tracker where you want it, the following controls will allow you to control the track:
The square will stop tracking if it is in progress, the play will track from the current frame until the end, while the button in between will track the next frame. The buttons on the other side of stop do the same but in reverse.
To assign something to the track, create a transform node and click the button next to translate to open the keyframe menu. Go to ‘Link To’, then the tracker you wish to follow, then the name of the track to assign the transformation to the tracker