The number (not name) of a pin or a pad is at the heart of how parts of KiCad work… although the name is important in some contexts.
This page has been checked to see that confusion in my brain (before 9 November, 2011) about the roles of pin names and pin and pad numbers hasn't led to errors here. I have a page with more on this if you are really curious.
You may also find times when you are confusing a schematic symbol or footprint's name with a pin or pad name/ net name.
Each pad of a footprint (also known as "a module") has a "number", which is the identifier which links it to a pin of a schematic symbol on the project's circuit diagram in "ordinary" situations. (It is only when you using power ports and invisible pins that things get a little complicated.)
Pin and pad numbers may be numbers in the usual sense, e.g.1, 27, 9999. However, the "numbers" can have letters in them, too, not just digits. "Vcc" is the "number" of an invisible pin on some devices, for instance. Nothing other than letters and digits should be used in a pin or pad number.
(Each pin also has a name, and each pad has a net name, but these are used for other things.)
Invisible pins offer a little challenge. What is their number (and name)? Often you will "just know". There is help with how to find the pin number (and name) at the invisible pins page in this encyclopedia.
Pin/ pad number rules….
(You'll need this information when making your own schematic symbols and footprints, and even before that you may find things here useful.)
Maximum length: Four characters.
If you try to assign a longer name, KiCad just abbreviates what you've used.
(Minimum length: While the schematic symbol editor will let you make a pin with no number, this seems like a Very Bad Idea to me, and I have no idea why you would want to create such a pin, and the fact that you can do it at all puzzles me. (I should go back and re-run the tests which made me think it is possible!) Pins without names is a different matter. They do occur.)
Spaces: The eeSchema manual says "Avoid spaces in pin numbers and names". This would imply that you would also not want to use spaces in pad numbers.