Recently this button to optimize PHP's OPcache was added to Laravel Forge.
If you were wondering what PHP OPcache is all about and what pressing this button does with your application, read this article Olav van Schie wrote on the subject a while ago.
Every time you execute a PHP script, the script needs to be compiled to byte code. OPcache leverages a cache for this bytecode, so the next time the same script is requested, it doesn’t have to recompile it. This can save some precious execution time, and thus make your app faster (and maybe save some server costs).