 Most two-channel oscilloscopes can make continuous mathematical calculations using the input signal values and the results are displayed as an additional waveform. The aptly titled math menu here lets you add, subtract, or multiply one signal by the other. Those basic operations alone are quite useful, but on newer scopes you'll usually see an FFT or fast Fourier transform option as well. This mode continuously analyzes the frequency content of one channel signal and displays the results in that blue graph you see at the bottom. The vertical axis is amplitude, but the horizontal axis represents frequency content. Watch as I add another sine wave to the signal. You can clearly see a new spike indicating a separate frequency appear on the graph.