VSWR (Voltage Standing Wave Ratio) is a measurement of how well matched a radio transmitter and transmission line is to a load such as an antenna. In general, a lower VSWR means a system and load is well matched with most of the power being radiated by the antenna and a relatively small amount reflected back into the system.
When a system is not well matched to the antenna being used, standing waves can be generated with power then being reflected back into the system. If care is not taken to mitigate this reflected power, sensitive components such as power amplifiers can be damaged and render the transmitter unusable.