The first U in U=U stands for Undetectable. It's a word describing a measurement — like saying “foot” means 12 inches. Or a bushel means 32 quarts and a peck is 8 quarts. So when someone loves you a bushel and a peck, that's 40 quarts of love there.
But back to U... When a person living with HIV has a viral load that falls below 200, it goes into the “undetectable” range. And when your viral load is undetectable, there simply isn't enough HIV in your body to infect somebody else when you have sex with them.