refers to the removal of dead or spent flowers either to encourage more flowering or to improve the general appearance of the plant. Most annuals and many perennials will continue to bloom throughout the growing season if deadheaded. Rudbeckia and Echinacea are good examples of perennials that benefit from deadheading.
Deadheading can be accomplished by pruning the spent flowers or even by pinching them off with your fingers. For plants with a profusion of tiny flowers, like threadleaf coreopsis, it is often easier to deadhead by shearing back the whole plant. It will take a little longer to set new flowers, but it will.
Sometimes it seems tedious to deadhead my flowers, but the new blooms make it worth it.