I was able to find this from the National Weather Service:
Most of the time, the forecaster is expressing a combination of degree of confidence and areal coverage. If the forecaster is only 50% sure that precipitation will occur, and expects that, if it does occur, it will produce measurable rain over about 80 percent of the area, the PoP (chance of rain) is 40%. ( PoP = .5 x .8 which equals .4 or 40%. )
In either event, the correct way to interpret the forecast is: there is a 40 percent chance that rain will occur at any given point in the area.
Now I’m not sure why it is shown on the hourly forecast as not having a percentage, but I’m guessing it could be due to an anomaly or bug. Maybe someone else on here can explain it in better terms.