For the purpose of this tutorial, I am going to use Pacifico, a font you can find on Font Squirrel.
A custom font is a font that is not of a standard nature (i.e. one that comes with your computer when you buy it), nor is it a Google font, which are readily available for use on the web.
A custom font is something you download from websites like the one mentioned above, purchased from designers or any other method you may use to obtain fonts that are not considered to be web fonts. Typically, people use them on images only.
This tutorial will teach you how to install a custom font on your website in under five minutes and as a result, will open so many more options for you and your web design.