Is there a list that explains what each icon means in Motion?

There is not a single listing in the app for telling what the icons mean, often times they are sprinkled in the documents and sometimes they could be obscured by disclosures in the documents. Some are pointed out specifically, for example the Rasterization section>"Intro to rasterization" calls out elements in the Inspector or layer list that help explain when rasterization (basically hard comping of layer elements) is occurring. Generally this is when an icon in the layer list has a square around it. Not every icon has a tool tip to tell you what it means.

I'm old, and always recommend reading through the docs over a few weeks/months to get to really know the software. Doing this will help you pick up these finer details. If there is a particular icon you are not sure about, just screen shot it and make a post, someone here will know. 😁

The "thumbtack" allows you to *pin* an object's parameters to the Inspector. When activated ("down"), the parameters in the inspector will stay even if you move off the current object and select something else.

Let's say, you want to be able to control one object's parameters, but you need to move an object in the canvas. Clicking and dragging the canvas object will usually automatically move the selection to that object, but if the thumbtack is activated, the parameters of the previously select object will stay in view.

Another way to use this feature is to edit two sets of parameters at the same time without having to go back and forth between objects. You can Pin one set of parameters for one object, then select another object and type "D" to call up the object's HUD display — and there you go — two sets of parameters that can be edited at once (this depends a great deal on which parameters you're actually editing because not all parameters are available from the HUD.)

This "feature's" usefulness, from my point of view, is only marginal and I might use it once in a blue moon. I'm sure some use it as part of their regular workflow...

Move the mouse over an icon and pause for 3 seconds... a small popup will appear:

(I paused the mouse over the gear icon)