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.).
MNE offers several types of sensitivity map (--map). Options 1 and 2 are the most useful for most researchers. Option 1 provides a measure of sensitivity independent of source orientation. Option 2 provides a measure of sensitivity to sources normal to the cortical surface. Please refer to the MNE manual for more details.
The following publication provides more information on the sensitivity of MEG and EEG measurements:
Goldenholz, D. M., Ahlfors, S. P., Hamalainen, M. S., Sharon, D., Ishitobi, M., Vaina, L. M., et al. (2009). Mapping the signal-to-noise-ratios of cortical sources in magnetoencephalography and electroencephalography. Hum Brain Mapp, 30(4), 1077-1086.