Treatment as prevention reduces the likelihood of HIV infection through sex if one partner is HIV positive and the other partner HIV negative.
This method consists in the HIV-positive person taking HIV medication that permanently supresses HIV replication. If, over a period of at least half a year, HIV can no longer be detected in the blood, then the viral load has become very low also in other body fluids such as sperm and vaginal fluid. If this is the case, HIV can no longer be transmitted through sex.
You can then have sex without a condom and still be protected against HIV. In other words, treatment as prevention is also a method of safer-sex.
Important: Medication must be taken regularly and its effectiveness must be regularly checked by a physician.