A term with two definitions - one more drastic than the other.
The first definition - involves burying a sharp, flat shovel to its blade depth in a circle around a plant, typically just beyond the drip line. This action severs the roots and suckers that extend beyond the pruned circle, and may encourage more top growth. Suckers (if true to parent) may be dug up and transplanted elsewhere.) Such pruning on an annual basis can help keep vigorously spreading plants in check.
The second definition is a tongue-in-cheek euphemism for digging up and discarding a plant that is not performing as expected, or is diseased.
Many gardeners can't bring themselves to discard a growing plant, even if it isn't performing up to par. But at some point, most gardeners are faced with the tough choice - especially when space is at a premium, and a new variety or plant is desired in the spot occupied by a less-desirable plant or shrub.