The peaks are the frequencies at which the vibration amplitude is maximal. Here, they appear to be at about 900 Hz, and about 1200-1300 Hz. The peaks are about the same in both experiments, however greater in the 400 N load.
If you want to find the amplitudes, use the Signal Processing Toolbox findpeaks function on both experiments. See the documentation on findpeaks function for all the options the function has.
