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CMB lensing is the gravitational lensing of CMB photons by high-density regions of space. Mapping of the density of the universe is being attempted through analysis of the CMB, looking for and analyzing any 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 that a presumed dense region would create.
Delensing is figuring out what the CMB would look like if there were no such lensing. One method uses the cosmic infrared background (CIB). CIB sources, to a large extent, are the same dense regions as those producing CMB lensing, offering independent information on how the CMB is likely to be lensed.