Our main KB overview article is here:
You can use that to search for different tutorials including the Forn Manager:
Think of the Font Manager as a placeholder. You add a unique name for your font. You go to X > Launch > Templates > Fonts and add a name with a font assigned to it. For example MyMainFont.
Now in the different parts of the website including the typography options in X > Launch > Options > Typography you have a choice to assign that name to the body or header section of the website. Body represents the normal text in content areas and Header represents the titles such as H1 H2, ,,,
The thing that Font Manager does is from now on you can use that in different elements too. For example, in the new Text Element inside the Cornerstone, you can use the same Font Manager name to add the font to the text. Now the cool part is that for some reason if you want to change the font of the website and the places that you used, you just need to go to the font manager and change the font assigned to the MuMainFont and the whole website will be changed and you do not need to go one by one to options, different element to change the font.
That is the whole goal of the Font Manager.
Now regarding the custom fonts. There are no options available in our theme for that and you need to add the font like any other website using the CSS and assign the font to different parts of the website using CSS code.
You can switch off the font manager and use the actual font name such as LATO in the options or different parts of the website, but if you want to change the whole font of the website at one point you will need to go one by one to all those elements and options to change it from LATO to Open Sans for example.
The Font Manager has a list of fonts which you can assign to the MyFontName which all are Google Fonts. If you want, you can assign a Typekit font to it too if you have the Typekit account. Kindly read the tutorial I have sent on how to setup the Typekit to be available in the list of the font manager to choose from.
Hope I could explain the relations correctly.