When you are in Compose, the Item List changes and has a lot of new features.  Here is the rundown on the features of the List.

Character Groups are collections of items ( objects, channels, lights, cameras, etc.) that you want to group together.  For more information about Character Groups, click here.  If you don't want more information about Character Groups, click here.
Tracks are like a "rack" that you hang Clip Instances on.
Clip Instances are references to the actual Motion Clips.
Pinned/Normal (unpinned) When a Clip Instance is "pinned" it will stay on the Track Timeline even when you are working with a different Group.  This can be useful if you need to match the timing, or at least know when a Clip on a different Group.  The pinned Track/Clip Instances will not affect the motion of the active Group, they're just there for visual reference.
Unlocked/Locked is

1) Processing Order (sometimes called Solving Sequence or Solving Order) refers to when you want the motion to be applied.  The choices are Apply After Motion, which will let all motion on the group calculate first, then apply the motion from the Clip Instances; Apply Before IK, which will apply all of this before the IK is calculated; and Apply After IK, which will apply it after the IK calculations are made.

2) Motion Active/Bypassed - Use this toggle to allow the motion to be used, or to bypass it.

3) Motion Composite Operator - Choose how you want the motion to be applied with regard to any existing motion that is already on the item.

4) Motion End Behavior - (also called just "End Behavior")  Choose what you want to happen at the end of the motion in the Clip.  The choices are:

• Repeat: The motion will repeat by looping back to its first keyframe and starting over.

• Oscillate: The item's motion will loop by playing forward and backward over and over.  Also called "ping ponging."

• Accumulate: The motion will be added to in successive loops.  For example, if you have a ball bouncing, it will bounce progressively higher, like it is going up stairs; even the bottom part of the bounce will get higher.

• Reset: At the end of the motion, it will go back to it's first keyframe position and stop.

• Stop: Stops at the last keyframe and stays there.

5) Motion Compensation -  There's no particularly easy way to explain this, but fortunately, you don't necessarily need to understand it, because you'll instantly see (as you scroll through the choices) what works and what doesn't.  But here's an attempt at explaining it:

When you apply a Clip to a character, you need to be able to decide if, or how, to compensate for animation that may exist on the objects.  The "if" is in reference to whether the character and its components are situated at their rest positions (or not) at the frame you want to insert the Clip.  Motion Clips contain not only the keyframe information, but also the Rest information for each item.  

=  No compensation.

 c  Use the Rest position(s) contained within the Clip.

 g  Use the Rest position(s) that the members (items) in the Group are set for in the current scene.

ck  Use the first keyframe of the Clip as the Rest position(s) and calculate the motion in the Clip relative to that.

gk  Use the first keyframe on each member (item) in the Group as the Rest positions(s) and calculate the motion in the Clip relative to that.