Merges all open tabs into one bookmark folder to be restored later. Just like all other bookmarks, MergeMarks are synced and backed up by your browser (if enabled).
This is very similar to the great OneTab extension (https://chrome.google.com/webstore/detail/onetab/chphlpgkkbolifaimnlloiipkdnihall), with the difference that sessions saved with MergeMarks are synced and backed up with your browser bookmarks.