The document discusses implicits in Scala. It provides examples of how implicits are used in libraries like Akka and Finagle to enable loose coupling and dependency injection. Implicits allow passing values without explicit references. The document also discusses type classes and how they can extend libraries using ad-hoc polymorphism. It warns that implicit resolution rules can be difficult and implicits should not be overused or over-applied. A demo is provided showing implicit conversions, parameters, classes, and scoping rules.