Alex's answer says it succintly, but it may not be clear to everyone, given the comment above:
"Resource" is how developers want to refer to their "User" Model
The authors of devise realise that those who use the gem will want it customised differently. Some people will want to call their users: Users, others will want to call them: Swimmers, or Golfers, or Presidents, as the case may be. "Resource" is basically a substitute for the name of the users that app developers will utilize in the future. Devise doesn't care what users are actually called: no matter what it is called, to devise, your users will be known as simply "resource". If it had been any other way, then the authors of the gem would have to force app developers to call their users: Administrator, or Admin - which is very restrictive and developers will not like that.