Default means something is automatically set up a certain way.

The default font is the font that all new documents are automatically set up to use.

The default font for documents created with Google Documents is usually set to Arial.  Arial is nice because it is simple and professional.  It's not too fancy, and easy to read! 

Sometimes, you'll change the font in a document to make it look different.

It's important to think about who will be reading the document before you change the font.

For example, you wouldn't choose a cartoon-type font if you were writing a research paper for school because a cartoon-type font looks unprofessional.

Fonts that look professional are easy to read, usually look fairly simple, and don't look like handwritten text or strange symbols.