Playables can be used to create complex and flexible data evaluation trees. They can be connected together (via Playable.Connect), after which each Playable in the tree can set the "weight" or "influence" of each of its children. Once a tree of Playable is created, it can be "played" by using it with a DirectorPlayer component. (see DirectorPlayer.Play)
Once a Playable (or tree of Playable) is associated with a DirectorPlayer, the player will traverse the Playables one by one. Each time it encounters a Playable in the tree, it will call its PrepareFrame method. This allows the Playable to "prepare itself for the next evaluation". It is during the PrepareFrame stage that each Playable can modify its children (either by adding new inputs or by removing some of them). This enables Playable to "spawn" new children branches in the Playable tree at runtime. This means that Playable trees are not static structures. They can adapt and change over time.
AnimationPlayables are a good example usage of the Playables. They allow existing AnimationClips to be connected together via AnimationPlayableMixers, on the fly. This can be used to programatically create animation behaviours such as locomotion blendtrees. Once a tree of AnimationPlayables is created, it can be passed to an Animator component via the Animator.Play method.