They developed a piece of mRNA that produces a specific type of blood protein that binds the HIV infection mechanism. The bound protein triggers your body’s antibody cascade to attack and neutralize it.

From TFA to set context: The vaccine is an immune primer, to trigger the activation of naive B cells via a process called germline-targeting, as the first stage in a multi-step vaccine regimen to elicit the production of many different types of broadly neutralizing antibodies (bnAbs). Stimulating the production of bnAbs has been pursued as a holy grail in HIV for decades.

That is, the vaccine triggers generation of bnAbs successfully, turning a "passive immunization" HIV/AIDS treatment in to "active immunization".

More on bnAbs:

> Antibodies are proteins that immune cells make to block viruses and other infectious agents. In the case of HIV, people who are infected typically produce antibodies to the virus. But because the virus mutates and replicates rapidly, antibodies are largely ineffective at controlling the virus. After years of infection, though, some people produce highly potent antibodies called broadly neutralizing antibodies (bnAbs) that, in laboratory tests, are able to neutralize a wide variety of HIV strains. The identification of such antibodies has transformed the field of HIV prevention research for two reasons: it provides information to guide the design of vaccines that could elicit bnAbs for protection, and it has opened the door to a new prevention modality: the administration of HIV bnAbs to prevent infection.

> The administration of antibodies to prevent infection is known as passive immunization, in contrast to active immunization, which occurs as a result of vaccination (see graphic, below). While a vaccine “trains” the immune system to generate antibodies and other immune responses, passive immunization requires that the antibodies be delivered directly into the body through infusions or injections. This protection is temporary, and, in the case of HIV prevention, would need to be administered periodically as long as the subject was still at risk.

From: https://www.iavi.org/our-science/bnabs-for-hiv-prevention