Elementary, like Debian and some other Debian derivatives, doesn’t support PPAs out of the box. In order to use them, you’ll have to install `software-properties-common`.
Open your terminal with CTRL + ALT + T and simply enter the following:
sudo apt install software-properties-common
Once you’ve done that, you can add PPAs in the normal way. For example, this is a LibreOffice PPA:
sudo add-apt-repository ppa:libreoffice/ppa