hiv positive
If your HIV viral load is undetectable, then there's not enough HIV in your system to transmit it. Out of over a hundred thousand people with steady partners that they've tracked with this, there wasn't a single case of transmission.
The HIV-negative partner is trusting that the HIV-positive partner is being honest, and being vigilant about their own healthcare. The "undetectable" status is not a permanent thing; they have to keep up with their meds and stay under medical care to be sure the meds are still working. Sex with someone you don't know and trust is always a risk. You can reduce that risk with the sorts of precautions you can control - condoms, or changing what you do, or taking HIV drugs yourself as pre-exposure prophylaxis, what's commonly called PrEP.