Star/favorite goes back to the original poster but isn’t otherwise visible. A nice way to say hey, I liked this! (Addendum: Today I saw a star with a value on it on someone’s feed, so it’s possible this is a feature on some Masto instances.)
Boost=share to spread the post.
There is no “boost with commentary.” You either boost or you add commentary. Put your commentary as a reply (and mark the reply as unlisted, see below).
Use hashtags, which you can also follow.
The globe icon helps you set visibility. A good guideline is global for top level posts, and then unlisted for the thread.
You have two feeds: Local and Global. Local is filtered to your mastodon server (so, like “just things from mastodon.social” for example). Global is your local feed plus anyone you follow, including in other servers.
CW is “content wrapper.” Folks are more aggressive about using them for a lot of things, even stuff that doesn’t usually get cws else-net. (There’s controversy over using CW for encounters with bigotry, especially for BIPOC folks, and the fallout of that will hopefully be to not feel obligated to use them for your own lived experiences with prejudice/bigotry.)
And add alt text to all your pics. If in doubt, imagine you are providing the audiobook narration for a really good comic book.
As of Mastodon 4, I believe, there is an edit function. Not all apps have this yet, but it’s available on the web interface. As with all things in Masto, your instance may vary the rules a bit.