This is how you 'flag' a film or, clicking on it again, 'unflag' a film. You can then view your flagged films at any time by clicking on Flagged Films in My FWFR.
What purpose you have for flagging films is up to you. Some people use it to mark films they need to review to complete an accolade, others, me included, use it to remind them of films they want to watch.