TabSnap essentially allows you to take a "snapshot" of the tabs you have open in Safari at any given time, and then open that set of tabs whenever you choose. You can instantly open a previously saved set of tabs, overriding the tabs you have open currently. Don't worry about losing your existing tabs, though. Whenever you restore (open) tabs from TabSnap, it will automatically create a backup of your current tabs so you can restore them later. TabSnap also lets you name the file for the set of tabs that will get saved so you can easily find it later. If you don't give it a name, it will be the timestamp.
How is this different from Safari's built-in technologies? Safari maintains a folder deep within your user directory called "SavedState" which keeps track of the windows and tabs that you had open. This allows you to recover your open web pages when you relaunch Safari. However, this only lets you restore your last session. What if you want to restore the tabs you had open two sessions ago? How about ten sessions ago? If you saved that session using TabSnap, you can. TabSnap keeps and manages snapshots of multiple sessions, so you can easily recover any old tabs. Let's say you routinely visit certain websites in the morning, and then visit different websites when you get home in the evening. Just save your morning tabs and your evening tabs, and seamlessly switch between the two sets of tabs. Although this sounds like bookmarks, TabSnap instantly opens all of the tabs that were saved at the time. You don't need to open bookmarks one-by-one to access your favorite web pages. It compliments the "pinned tabs" feature available in OS X El Capitan (10.11) and later.
Other Information At this time, TabSnap only supports Safari. If the TabSnap files are starting to add up and you want to delete some, you can open TabSnap and go to More > Preferences > Delete saved tabs, and choose which files you want to delete.
Instructions After TabSnap downloads, you can optionally move it to your Applications folder, then launch it. If you get a message that it is from an unidentified developer, right click on TabSnap and click Open and TabSnap's main menu will open.