You can enter several fonts, separated by semicolons. LibreOffice uses each named font in succession if the previous fonts are not available.
Any font changes apply to the selected text or word in which the cursor is positioned. If no text has been selected, the font applies to text typed afterwards.
The last five font names that have been selected are shown in the top part of the combo box.
In LibreOffice you see the available fonts only if a printer is installed as the default printer in your system. In order to install a printer as the default printer please refer to your operating system documentation.
You can see the name of the fonts formatted in their respective font if you mark the Show preview of fonts field in LibreOffice - View in the Options dialog box.