You may have noticed that some of your Facebook friends changed their profile pictures to the colors of the French flag. In the aftermath of the terrorist attacks around Paris, Facebook quickly created this filter to let users show support for France and its people. This change is similar to the rainbow profile pictures that celebrated gay marriage in June.
There are two ways to get the flag gradient. One is to simply look at your newsfeed and find a friend who has changed their profile photo and click the "try it" button. The other is to go to Facebook's own page, find the pinned "We stand together. #JeSuisParis" post, and click the "try it" button.