CMB Lensing is the Gravitational Lensing of Cosmic Microwave Background 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.