As the name suggests, marks work like bookmarks for the command line.
By default, these are inserted automatically any time you press Enter.
you could jump between the automatically inserted marks by tapping Command+Up or Command+Down.
The default behavior isnt for everyone though.
You might find them more useful on the manual setting.
Head over to How-To Geek for a bit more on using marks in the Terminal.
How to Use Marks in OS Xs Terminal for Easier Navigation| How-To Geek
