Step 1: Open SubWallet homepage and choose the Settings tab on the sidebar.

Step 2: Scroll to the end of the Settings section to see the "Lock" button. Then click the button to lock the wallet.

If you have previously locked the wallet, or if you have been inactive for a while, your wallet will be locked.

To unlock the wallet, you would need to enter your password and click "Unlock".

More on the password can be found here.