I've heard a lot about it, but I do not understand how it works?)
It's simple, you install the MetaMask extension in your browser of choice, be it Chrome or Firefox. Then you can create an address in MetaMask that you can use or you can import an existing one. Once you've created an address, websites can ask MetaMask to sign transactions.
It's a bit safer than using EtherDelta's built in wallet system because if the DNS was hacked and redirected to another site, it would have to request you to sign the transaction using MetaMask and you'd be able to see if it was doing something different to what the malicious EtherDelta said it was doing. I.e depositing all of your Ether into another wallet which is not the action you were taking on the website.
With that said, there are some risks with using MetaMask as well. So make sure you don't put that much money into your MetaMask address.
how about electrum? is it safe?