This means that you have antibodies in your system against the hepatitis B virus. If you have had the hepatitis B vaccination series in the past, your test result shows that the vaccine was effective-it caused your body to create antibodies against the virus, that is you're immune.
If you did not get the vaccine and don't recall ever being diagnosed with hepatitis B in the past, it means you have had a hepatitis B infection in the past and your body responded by creating Hep B antibodies.
Many times infection with Hep B is "sub-clinical", meaning that some persons exposed and infected with it do not have severe enough symptoms to seek medical care. Sometimes people who think they had a "touch of the flu" actually had a Hep B infection. Unless they went to seek medical care and got tested they would not know they ever had a Hep B infection.
The Hep B surface antibody test is done to diagnose people who have had Hep B in the past or, in people who have received the Hep vaccine, it is used to assess their "immune status" to see if they had adequate antibody response after the Hep B vaccine.
Hep B vaccine is among the routine childhood immunizations for some years now. So checking the Hep B AB (AntiBody) level is done in health care workers in job application process, entering college students, armed forces entrance, etc. to ensure active immunity.
If immunity not adequate some recommend getting a booster vaccine.
Hope this answers your question.
I'm a doc