Resident Agent, Defined

A resident agent also know as a registered agent or a statutory agent, as used in United States business, is a business or individual designated to received service of process (SOP). Which means that a state will/shall have within its' borders a PHYSICAL address, (not a PO Box), phone number and at least one person designated to receive served papers, materials, information and in some cases all your renewals, tax requirements, etc. as if it were you/your company office. To some states it is, as they WON'T send the material outside their borders. In which case you are left in the dark without them.
In short their purpose is to keep you informed in your relationship with that state/jurisdiction. As it has been put; failure to properly maintain a registered agent (or its' other names) can affect you/your company negatively!!
Be prudent about who/whom you choose!