If someone is HIV positive and is taking medication to treat it, then the level of HIV viruses in their blood can drop to basically zero. This is called an undetectable HIV load. The person still has HIV, and if they stop taking medication the virus will come back.
If an HIV positive person is undetectable and on medication, they cannot transmit HIV to anyone else. This has been studied in hundreds of thousands of couples, and is now a well established medical fact. (more info on TasP).
Some people might tell you that they’re using TasP for prevention, which means that they’re monitored by a clinical team, they’re on their medications, and they cannot transmit HIV. Communication is key!