Hey @Aramesh, thanks for asking 
MetaMask Flask allows developers to extend and customize MetaMask with their own features and experiments they develop, and users can download MetaMask Flask to use these features as well! But as noted:
If you’re not a developer, we recommend you only use the main MetaMask applications and do not use MetaMask Flask. If you’re a web3 dapp developer and feel adventurous, then MetaMask Flask is for you.
Here is more information on the MetaMask website if you are interested: