Each directory, except /, is inside one other directory. That directory is its parent. The name .. means the parent of the current directory.

Go up with cd ..

learner@tidepool:~/shop/orders$ cd ..
learner@tidepool:~/shop$ cd ..
learner@tidepool:~$

Put two together to go up two levels: cd ../...

Use .. in a relative path

.. is a name in a path, the same as any directory name. Go up, then go down again:

learner@tidepool:~/shop/orders$ cat ../prices.txt
tea 4.50
coffee 3.20

The top of the tree

/ has no parent. At /, .. is / again. You cannot go above the root directory.