Sensitivity maps tell you how sensitive your channel configuration is to different parts of the brain, independently of source estimation. The MNE function mne_sensitivity_map compute the signals of point sources all over the cortical surface (i.e. at all vertices in source space), and plots the resulting signal amplitude at the surface at the corresponding source location on the cortical surface. The larger the value, the more sensitive you are to this particular source.
Sensitivity maps have to be computed for different channels types separately (i.e. for magnetometers, gradiometers, and EEG), because their measurements are in different physical units (fT, fT/m, and µV, resp.).