Remembers the last viewed post on your Tumblr dashboard in case you accidentally navigate away
In the same tab go back to Tumblr and left click the Remembr button, it will start searching for the last post you had open in THIS tab.
Just open a new tab/browser and click the Remembr button, this will show a popout of possible last posts (one for each tumblr tab you had open).
Alt+Click the Remembr button and it will show the possibilities.
Alt+Click the Remembr button to show the list of possibilities and click the 'x' item in the list. This will ask you to confirm that you want to delete all remembered posts.