U=U means Undetected = Untransmittable. It is part of an international campaign to raise awareness of HIV treatment. If someone's HIV viral load can't be detected in their blood, it is impossible for them to transmit HIV to someone else. The campaign focuses on spreading awareness to reduce stigma around HIV.
We wholeheartedly support this campaign and you can too, with this awesome cap, the colour of which is officially 'biscuit'.
Photo by the talented irl subscriber Gaby Barnes! (@gabyreads1708)