What is Value Stream Mapping?
A value stream is a process that adds value to a collection of things that have less inherent value.  The canonical example there is an assembly line.  I have little interest in a disconnected collection of parts (steering wheel, shifter, gas pedal.)  On the other hand, I'll pay good money for a car.  Car manufacturers add value to the parts in the form of facilities, skilled work, supply chains, etc.  Parts go in one end of the stream, cars come out the other.

Enterprises use complex processes to deliver their products and/or services.  Value stream mapping is a process used by companies to figure out how and where they deliver value.  The purpose of value stream mapping is to develop a clear understanding of the steps, time, and resources required to support a business capability that delivers value.  

Value stream mapping is useful in kaizen, or Continuous Improvement.  The "systems thinking" employed in DevOps and Lean encourages small, incremental improvements made throughout a complex process.  A value stream map can expose areas of waste, such as unnecessary or inefficient steps in a value stream.  A complete map tends to expose those areas of waste.  This helps prioritize improvements.

Value Stream Mapping can be applied at large scope to a business process, or to a smaller scope within software delivery.  The software delivery process itself mirrors an assembly line.  Code is written, reviewed, tested, integrated, and tested again.  It is delivered from one environment to another (in manufacturing this is called a work center) until it is finally delivered to Production.