A Google image search of the icon wasn't super helpful, but did return a similar-looking image named "style" among the results.
Knowing that Liftoff is written in Flutter, I confirmed that it is called the "style" icon.
A search of the Liftoff code on GitHub for the "style" icon turned up a match with the comment:
// TODO: change styles?
So the answer is that it does nothing now, but appears like it might be used in the future to "change styles." Maybe as a shortcut to the "Post Style" screen in "Settings."