Web sites and apps seem to have a tendency to change features as they go along, such as Facebook now "auto playing" videos on your timeline when you pause for more than a microsecond (it seems).
These changes are of course generally designed to "engage" you, or otherwise get you to click on things, and so on - all part of keeping eyeballs engaged and making ad revenue. Nothing wrong with that in itself, but sometimes the methods get in the way of your enjoyment.
The Facebook video thing can be annoying if you are just trying to quietly check your feed, and suddenly a video of a dog barking or a marching band playing (or whatever) shatters the silence. Also, you may not want videos playing on your mobile device, using up some of your data allowance for the month.
HowToGeek has simple fixes for both PC and mobile users who don't want Facebook to play timeline videos unless you actually click on them. Get the skinny here.