You’re right, undetectable is a word that we use to describe when someone on HIV medications has a very low viral load.
If someone is HIV positive they can manage their health by taking HIV medications daily. HIV medications don’t eliminate the virus entirely, but they will keep the virus in check and lower the amount of virus in the body (viral load.) The lower someone’s viral load, the less damage the virus can do to their immune system, and the lower the risk of transmitting HIV to someone else.
The goal of HIV treatment is to lower the amount of virus to the point where the lab can no longer detect it in a blood sample, we call this “undetectable”.
It has now been scientifically proven that if someone living with HIV maintains an undetectable viral load (with the help of regular checks with their doctor and taking their medications as prescribed) there is effectively no risk of passing on the virus to a sexual partner. We call this undetectable = untransmittable, or U=U.