It depends: if you believe you are not infringing or violating another's copyright, you could explain your position to the person claiming infringement (or their attorney) and see if they agree; it would be better to retain an attorney to help you to this. If you are infringing, you should stop the infringing action--and again, if you are not sure if you are infringing, speak with an intellectual property attorney. If you in fact are infringing, you could be sued for monetary damages or compensation and also potentially face a court order requiring you to stop the infringing activity; therefore, you have to take this notice seriously.