You can find a good explaination on the web site of the Center for Space Research (University of Texas at Austin): http://www.csr.utexas.edu/projects/rs/whatissar/rar.html
"A radar can only distinguish the returns from various targets based upon the arrival time of the received signals. A nadir-looking radar could not distinguish between two scatterers a and b (see Figure 2.2) that are equal distances from the sensor because a single incident wave front illuminates both points at the same instant, so the scattered returns from both points arrive at the receiving antenna simultaneously. This leads to an ambiguity in range for any right/left-symmetric, equidistant points. If the radar illumination is restricted to one side of the platform, the wave front illuminates the same two points at different times. Their scattered returns arrive at the sensor separated in time and are thereby distinguishable from each other."