The Tree View is where you determine what types of actions you want to perform during your sequence.
Above we have a Camera Cuts track for switching camera angles during our sequence, a Cine Camera Actor to provide the view for a scene, and two Skeletal Mesh Actors which are the characters in our scene.
Sequencer is context sensitive and will allow you to add Sub-Tracks (where applicable) to each track that will provide you with additional options.
For example, in the image below we have added a CameraComponent track to our Cine Camera Actor which gives us the ability to add another Sub-Track to adjust some of the properties of the camera during our scene if we desire.
You can also add Keyframes from the Tree View for your tracks to define properties at a specific point on the Timeline.
You can click the + keyframe button or press the Enter key with a Track selected.