A spectrum analyzer is a piece of electronic equipment that is used to measure the magnitude (amplitude or strength) of a given input signal set against the full frequency range of the instrument. It is primarily used to measure the strength of the spectrum of known and unknown signals. The spectrum displays data in a graph, where the amplitude is represented on the y-axis and the frequency on the x-axis.
Techopedia explains Spectrum Analyzer
A spectrum analyzer looks very much like an oscilloscope; in fact, some models of this type of laboratory equipment function as both a spectrum analyzer and an oscilloscope. The spectrum analyzer, as the name implies, analyzes the RF spectrum, so the typical input is radio frequency and audio signals.
The spectrum analyzer can reveal details and elements about the signal that would otherwise be unknown, such as the efficiency and performance of the circuit or equipment producing the signal. It can perform different types of measurements, which means that it is an indispensable tool for designing RF-generating devices and for use in test laboratories and specialized field services dealing with RF waves.