surge region, located on the left-hand side of a compressor map (known as the surge line), is an area of flow instability typically caused by compressor inducer stall.
The turbo should be sized so that the engine does not operate in the surge range When looking at a compressor map, Compressor surge is when the air pressure after the compressor is actually higher than what the compressor itself can physically maintain.
This condition causes the airflow in the compressor wheel to back up, build pressure, and sometimes stall. Common conditions that result in surge are--

The compressor bypass valve is not integrated into the intake plumbing between the compressor outlet and throttle body

The outlet plumbing for the bypass valve is too small or restrictive or a turbo is too big for the application