A notice of opposition means a third party has formally objected to your trademark application at the trademark office. Their objection is that your mark should not be registered, and the most common reason is that they consider it too similar to a mark of their own.
Does an opposition mean I've lost the trademark?
No. An opposition does not end your application. To keep the registration process moving, though, you need to file a Trademark Defense, and it must be filed within the time limit set in the notice. Don't let that deadline pass.