. is the current folder.

.. is the folder above the current folder - the folder that contains the current folder.

You will sometimes see that single dot in use when someone wants to run a script from their home directory. For instance: ./install-app.sh. That means the file "install-app.sh" is in the current directory. It would be just as valid to do /home/username/directory/install-app.sh. The same way, you could also do ../install.app if the file is in the parent directory. The reason why it is this way, is not only for navigation, but also that it shouldn't be possible to accidentally hide system applications simply by misnaming a file in your home directory.