That's the shuffle button. It posts a random picture from your camera roll to a random community.

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."