Developer Mode allows to switch Surf to the Free TON devnet, a separate blockchain, running the latest stable version of Free TON, that is used for the purposes of development and testing.
Note, that while any Tonsyou have on your account won’t be available in the Developer mode, your address will stay the same. Only the network Surf connects to will be changed. If you want a different devnet address, you can log out of Surf (don’t forget to backup your master password for your original account first!), then create a new wallet and switch Surf to Developer mode to work with this new wallet.
Developer Mode can be turned on in Settings → Advanced settings → Developer mode. Turning it off works the same way.