As-Fan is a term that Wizards R&D uses to refer to the contents of an opened booster pack. To put it simply, it's the number of cards, on average, that you can expect to see in any given booster pack that fit the given criteria. An "As-Fan" of 2.00, for example, means that a pack will typically have two cards that fit the given criteria.
I primarily created this calculator for my own personal use. I use it for designing cubes and custom sets. I realized that there is also some value for knowing the as-fan numbers in limited formats, since it gives you a baseline on what you can expect to see in a draft pod.
On the pack and draft tabs you can also load numbers from officially released sets and draft formats to compare to your own custom sets or cube, or just for curiosity's sake.
It's similar to As-Fan, but rather than how many cards you can expect to see in a single pack, it's how many cards you can expect to see in a full draft (opened between all players).
It's the number of packs opened by a single player (3), all players in a single draft round (8), all players in 2 draft rounds (16), all players in a full draft (24) and a single player's sealed pool (6). All of the draft numbers assume an 8-man draft pod.
'Mechanics' in brackets are unnamed mechanics and themes I've included just for curiosity's sake. The amount of color-fixing in Return to Ravnica block, for example. These always appear after the named mechanics and you can hide them simply by changing the number of mechanics in the drop down at the top of the page.
It's used for cards that are always included in certain packs. Nonbasic lands in Dragon's Maze, Double-Face Cards in Innistrad, Timeshifted cards in Time Spiral, etc.
For creature types (for example, [Humans] in Avacyn Restored), I simply include any card with the creature subtype and any card that can create tokens with the subtype.