When something is undetectable, that means that it exists but is often unnoticeable. For example, a house could be so well hidden by trees that it's undetectable to cars that drive by. When someone HIV positive says that the amount of the virus in their body is 'undetectable' that means that it's too low for the laboratory to give an accurate count of exactly how much virus there is. But the virus is still present in their body and in their fluids. The risk of the virus being transmitted is lower but not impossible.
