It means you can trigger an investigation into abuse you suffered before 18 at any time up to your 28th birthday. Once you report it the DA has one year to institute an action against your aggressor.
It's not a crime, but a section that defines when a specific type of case can be prosecuted - an when it's too late.
The above answer is for general information only and is based on the information you posted. Every case is fact dependent, so to get a thorough analysis of your situation, you will need to consult face to face with an attorney licensed to practice in the jurisdiction where the incident took place. Do not conclusively rely on any information posted online when deciding what to do about your case.