The Allan and Allen surname derives from alluinn meaning fair, or handsome. It also derives from the Gaelic and Breton personal name of the pre-Christian era Ailin which loosely translates as Little rock, although it may also mean harmony.
The Allan surname spelled with an "a" is generally considered to be associated with Scottish clans, including Clan Donald, Clan Grant, Clan MacFarlane and Clan MacKay. Spelled with an "e," however, the Allen surname is generally considered to be English in origin. However, a variety of names from a variety of regions might be anglicised as either Allen or Allan, so the name spelling may not point to your family's origin.