What
Simple, sans Serif fonts like Calibri or Arial are easier to read and should be used for small text or large chunks of type. Serif fonts are more appropriate for adding visual appeal to large text like a title. Some users may benefit from specialty fonts like those made for people with Dyslexia. Text should not be too small to read easily: A good rule is no smaller than 12 pts for documents and no smaller than 20 pts for presentations.
Why
Choosing fonts and font sizes with accessibility in mind is especially beneficial to people with visual impairments or with certain learning disabilities such as Dyslexia, but making text easy to read at a glance makes reading faster and easier for everyone.