CMB lensing is the gravitational lensing of CMB photons by high-density regions. Mapping of the density of the universe is being attempted through analysis of the CMB to look for and analyze CMB lensing. The method is to produce a lensing map of the CMB, looking for deviations from its expected angular power spectrum that could reflect gravitational lensing by a galaxy cluster, etc. Analysis involves deducing the lensing deflection field created by a supposed dense region.
Delensing is figuring out what the CMB would look like if there were no lensing. One method uses the cosmic infrared background. CIB sources, to a large extent, are the same dense regions as produce CMB lensing, offering independent information on how the CMB is likely to be lensed.