What Does Synthetic Aperture Mean?
In general the larger the radar antenna, the better the spatial resolution.  It's prohibitively expensive to place very large radar antennas in space and impossible to put one on an airplane, so the spacecraft's motion and advanced signal processing techniques can be used to to simulate a larger antenna.
A SAR antenna transmits radar pulses very rapidly. In fact, the SAR is generally able to transmit several hundred pulses while a spacecraft passes over a particular object. Many backscattered radar responses are therefore obtained for that object. After intensive signal processing, all of those responses can be manipulated such that the resulting image looks like the data were obtained from a big, stationary antenna. The synthetic aperture in this case, therefore, is the distance traveled by the spacecraft while the radar antenna collected information about the object.
The ERS-1 satellite's SAR sends out around 1700 pulses a second, collects about a thousand backscattered responses from any single object while passing overhead, and the resulting processed image has a resolution near 30 meters. The spacecraft travels around 4 kilometers while an object is "within sight" of the radar, implying that ERS-1's 10 meter x 1 meter radar antenna synthesizes a 4 kilometer-long stationary antenna!