What undetectable means
The goal of HIV treatment is to reduce the viral load down to ‘undetectable’ levels. This does not mean that the HIV has gone from the body, but it means that the levels of HIV are so low that they can no longer cause any damage to the body, and that HIV will not be transmitted through sexual contact.
If you have been on treatment for at least 6 months and your viral load is undetectable then you are no longer infectious to others, as long as you continue to take your medication as prescribed.
For more information, see the Terrence Higgins Trust Can’t Pass It On campaign:
An undetectable viral load does not protect against acquiring or passing on other sexually transmitted infections or prevent pregnancy, so using a condom is still advisable in some situations,