What does undetectable mean? — Francesco Di Benedetto

and so it happened

Medical advances and the availability of PrEP have certainly improved and changed the lives of the HIV-positive community in recent years. Public attitudes, however, have not kept pace. The stigma endures and the need for better understanding remains. And so this project provides a safe platform where the portraits and the stories of those either living with HIV or on PrEP are shared, because openness is key to tolerance and no one should have to hide — whatever their status.

It means that the traces of the virus are so low that modern medical tests can not “find” HIV. It does not mean cured. It means not detectable.
A viral load test is a lab test that measures the number of HIV virus particles in a milliliter of your blood.
So, when someone says that they are undetectable, it literally means that they have reached a point at which their virus is not detectable by modern lab tests and they are not going to transmit HIV to sexual partners.
There is now evidence-based confirmation that the risk of HIV transmission from a person living with HIV, who is on Antiretroviral Therapy (ART) and has achieved an undetectable viral load in their blood for at least 6 months, is negligible to non-existent. (Negligible is defined as: so small or unimportant as to be not worth considering; insignificant.) While HIV is not always transmitted even with a detectable viral load, when the partner with HIV has an undetectable viral load this both protects their own health and prevents new HIV infections.
(from preventionaccess.org, and imstilljosh.com)