If you received a trademark notice of opposition, it means that a third party has challenged your mark and claims the Trademark Office should not register your mark. They have filed an Opposition before the Trademark Office. A common reason for opposition is that this third party believes your mark infringes on theirs.
A notice of opposition does not mean that you have lost your trademark, but you do need to present a Trademark Defense if you want to continue with the registration process. We recommend you retain a professional trademark service like Nominus or trademark attorney familiar with laws in your jurisdiction. Your counsel must present your defense within the period allowed by the Trademark Authority.