The browser extension allows you to quickly bookmark items and save links to your Wunderlist account from other websites, such as Amazon, YouTube, Gmail, and Outlook.com, or any other website of your choice. The extension is currently only available for Chrome & Firefox. If you're using Safari, then you should enable the OS X "Share" feature via the steps outlined, here.
You can install the extension for Chrome, here.
And for Firefox, here.
Once installed, you will see the Wunderlist icon just to the right of the browser’s URL address bar. Clicking on the icon will open the extension and ask you to login to Wunderlist. Once you’re logged in, you can add the current website title as an item and the URL will become that item’s notes. You’ll also see a dropdown menu where you can choose which list to send the item to.