Cabling refers to the use of cables to stabilize an established tree growing in a manner that is not sustainable, if left uncorrected. Cabling is often employed by arborists or other skilled tree service professionals to save a specimen tree. If cabling isn't done properly, girdling can result; cabling trees is not considered a DIY task. An arborist will know where and how to position the cables.
Cabling is sometimes used to save a tree with a split trunk, for example; without cabling, such trunks will eventually be torn apart. Another use of cabling is to support a large branch that is growing at an awkward angle.
Cabling is achieved by drilling holes in the trunks or branches of the tree in question, into which the arborist will insert the cable. The cable is secured so as to keep it tight. Don't confuse cabling trees with staking trees, an operation that involves anchoring the tree to the ground. Support in cabling is, by contrast, totally above-ground. Moreover, tree staking is meant to provide merely temporary support (for young trees), while cabling is meant to provide long-term stability.
"Guying" is yet another technique used to stabilize trees and can be thought of as a cabling method in which the cable is anchored to the ground (as in tree staking) or to another tree.