Just as the name implies, this is an image (right) of the estimated accumulation since the precipitation began. The accumulation continues until there is no precipitation for one hour anywhere withing the range of the radar.
Often, in prolonged rainy periods, this accumulation can exceed five days or more. On the radar page, the accumulation time period for this image is located on the right side, just above the image.
As in the case of the one-hour precipitation image the radar does a great job at correcting itself, there are times when the radar will be out of calibration. If the radar is "hot" (reporting echoes too strong) then the rainfall estimates will be an overestimate.
Conversely, a "cool" radar will underestimate the precipitation. Always check nearby radars to see if they are reporting similar information to what is viewed by your local radar.
Also, hail makes an excellent reflector of energy. Thunderstorms with hail will overestimate the amount of precipitation and the larger the hailstones, the greater the overestimate.