The document discusses concurrency and the actor model. It notes that as processors are no longer getting significantly faster, programs need to be parallelized to run faster. The actor model is introduced as a way to build concurrent programs where actors communicate asynchronously by message passing and have no shared state, avoiding issues like race conditions. Several Ruby libraries and frameworks for implementing the actor model are described, including Celluloid and Akka. The advantages of the actor model like encapsulation and decentralization are highlighted, though issues like potential for livelocks are also noted.