The Unpaywall button is an extension for Chrome and Firefox that indicates whether or not there is an open access, legal and free version of an article in a paid scientific journal. When you are browsing, it uses a padlock icon to show you if there is an open access version of the same article.
In other words, if you want to read an article from a journal we are not subscribed to, you can click on the button to see other open access versions of the same article in repositories or open access journals. The likelihood of finding an open access version is between 60-80%.
Install the extension on your browser. If you see a green padlock on the right hand side of your screen, this means there is an open access version available. If you click on this you will be taken directly to the document.