Why do you think the cations have an arrow and the anions have an indentation in the animation

The animation that you are speaking of shows how cations and anions interact in a chemical (ionic) reaction. The reason why the cation has an arrow while the anion has an indentation is because the atom (s) which formed that cation is giving up its electron to the atom (s) that forms the anion.

Thus the arrow from the cation indicates that electrons are being given up, while the indentation in the anion is indicating that electrons are being received.