At the start of a battle, before you have had a chance to act (specifically, before your first regular turn in the initiative order), you are flat-footed. You can't use your Dexterity bonus to AC(if any) while flat-footed. Barbarians and rogues of high enough level have the uncanny dodge extraordinary ability, which means that they cannot be caught flat-footed. Characters with uncanny dodge retain their Dexterity bonus to their AC and can make attacks of opportunity before they have acted in the first round of combat. A flat-footed character can't make attacks of opportunity, unless he has the Combat Reflexes feat.