A positive rapid test means that you swabbed your nose and found intact covid virus particles, and enough of them to trigger a positive test. A positive PCR test could still be picking up viral RNA after the virus was eviscerated by the immune system.

So a positive lateral flow = contagious, a positive PCR means maybe contagious. A negative of either means you could still be contagious, a negative PCR being much more indicative that you have cleared the virus than a negative lateral flow.