If you're a programmer, or at least someone who is very familiar with a command-line interface (DOS, UNIX, etc.), then this must be self-explanatory. But anyway, wildcards are special characters representing a certain set of characters or character groups, used if you're looking for a list of results following a certain pattern. For example, I * cheese means any phrase starting with "I" and ending with "cheese" and with anything in between. The wildcard '*' represents any group of characters, of any length.
Right now, '*' is the only wildcard you can use in Google. Since Google only recognizes whole words, you can't even use '*' to substitute parts of a word (e.g. M*cr*ft won't work), nor can you use the '?' wildcard...at least, not yet.
I can't wait for further developments in Google pattern searching. Today, asterisk wildcards. Tomorrow, regular expressions. The day after that, ZE WALD!!!
BTW, I like cheese. :-)