A friend gave me a backup he used for his Samsung Galaxy On5, How do I use these files to 'restore' my Samsung On5?
Boot in TWRP and hit Restore. It will show a list of backups, select one and you'll see a list with  System, Data, Boot. Make sure all the boxes are checked and swipe the bar to restore. The restore will take a few minutes, but when it’s finished, you can reboot your phone back into Android. If it asks you to root, be sure to tap “Do Not Install” if you don't wish to.

Flashing a zip file:

Tap on Install . browse to the location where you saved the file 2016-09-21_twrp_on5ltetmo_system_data.7z.
Now at the bottom of the screen, do a swipe on the Swipe to confirm flash option to begin flashing. Reboot the device when finished.