Google Chrome has already offered it for a while. Opera makes it extremely easy to disable plugins from F12. Now Mozilla Firefox is adding a click-to-play option for browser plugins.
The basic idea is to allow users to disable loading of plugins like Adobe Flash Player by default. Users can still click on the element to load the plugin and make use of them.
Plugins like Adobe Flash Player consume extra resources and can kill battery faster on laptops. HTML5 is becoming a very solid alternative for most of the things that required browser plugins in the past though it would still take many years before we can say goodbye to Flash in web browsers.