The document discusses concurrency in functional programming languages. It explains that there are two approaches to concurrency: mutable state and immutable state. Immutable state avoids issues with corrupt state that can occur with mutable state. Functional languages with immutable state and message passing, like Erlang, allow for easy distribution across computing resources from embedded devices to high-performance supercomputers. The document emphasizes how immutability, concurrency, and distribution enable scalability, reliability, and parallelism.