if a smell, taste, sound etc is masked by a stronger one, it cannot be noticed because of the stronger one:
Liz turned on a radio to mask the noise.
Air-fresheners mask bad smells instead of removing them.
to hide your feelings or the truth about a situation:
Men often mask their true feelings with humour.
to cover or hide something so that it cannot be clearly seen:
The new accommodation block has all but masked the original building.