What is Joule loss in distribution systems?

They are the losses defined by I ^ 2 R, for each section of your distribution. The current is always taken at true RMS.

Albert has answered you perfectly, but said in a less technical way are the energy losses in an electrical cable due to its resistance and which dissipate in the form of heat causing a (small) percentage of the voltage to be lost. For this reason, very high voltages are used in distribution to avoid having to transport many amperes (intensity) and in this way losses due to the Joule effect are minimized.