Complex sentence: If an LNA doesn't have high-gain, then the signal will be affected by noise in the LNA circuit itself; the signal may become quite attenuated, so the LNA's high gain is an important parameter.
Split into simpler sentences: If an LNA doesn't have high-gain, then the signal will be affected by noise in the LNA circuit itself. It may become attenuated, so the LNA's high gain is an important parameter.