If you have a sine-wave of 1Hz, it takes one second to “play” this sine-wave. If you have a second wave “shifted by 120 degrees”, this second wave will start exactly 0.33 seconds after the first, which is one third of complete time. The third wave will start shifted once again by 120 degrees, meaning 0.66 seconds after the first wave started.
This is required to control the three phases U, V and W of the BLDC motor at the right time.
Depending on the time sequence of the three phases, the motor will turn clockwise or counter clockwise.