Ankidroid -- What does the rectangular button in the action bar mean? I have wondered for a long time what this toggle  does. It saves some changes but doesn't indicate what has changed.
It allows you to "mark" a card.
It is a kind of bookmark that allows you to find this card easily later.

The most common case is when a card needs editing, but you do not have time/tools to edit it right now, so you mark it, and you will fix the card when you are back home.

Why not edit the card right away, instead of just marking it? Examples:


You are reviewing in the bus, and you notice a kanji that you probably entered wrongly, so you need to check. But you need the library's huge kanji dictionary, which is too heavy to carry in the bus.
You realize you made a typo when entering a LaTeX formula. You will only be able to fix it next time you are at your desktop computer.
You have a doubt about a card, and must ask a native speaker, but there is none around.


Reference: http://ankisrs.net/docs/manual.html#editing-and-more

The button now looks like a star, like in web browsers:



Thanks for using AnkiDroid!