Cabling trees is a process by which you use external supports to save an established tree that would not survive without them. It may have been damaged, or is growing in a way that means it needs external support. Tree cabling is done by fixing cables in exactly the right position to support the tree to continue growing.
The cables provide external support to prevent further damage which would eventually make a tree unviable, such as in the case of a split trunk. Tree cabling can also be used as a preventative measure. For example, if a branch is growing at a strange angle or too close to a building. Tree cabling can be used to manage the risk of it falling on the building or passers by. Tree cabling can also be used to preserve the overall shape and aesthetic appeal of an established, but damaged specimen tree.