Absolute vs relative path
Full address from / versus directions from where you stand.
An absolute path starts at the root and is unambiguous everywhere: /home/diego/notes.txt. A relative path is interpreted from the current working directory: notes.txt, ../photos, ./script.sh.
Two shorthands appear constantly: ~ expands to your home directory (so ~/projects is absolute in disguise), and .. means the parent directory. Scripts should prefer absolute paths: they run from unpredictable directories, especially under cron.
See it in practice
$ cd /var/log && pwd && cd .. && pwdAn absolute jump, then a relative one. Both end somewhere concrete: pwd always tells you where.
Where you'll meet it
Move between directories: absolute and relative paths, home shortcut, and jumping back.
pwdShow the absolute path of the directory you are currently in.
crontabRun commands on a schedule, and read the five-field syntax fluently.
Errors that come down to this concept: No such file or directory