Alternate file

Similar to how % represents the current file, # represents the alternate file, which is the last file you looked at.
If you run :ls and you've looked at more than one file, you'll notice a # mark next to one of the filenames. This indicates which file is the alternate file. To load the last file you've looked at you can use either of these two commands.
:e #

Further Reading

:h CTRL-^ :h alternate-file

More Gifs