A Value Stream describes a process, that is all the steps that take place from the initial request (or order or job or task or ..) to the final result (or product) within an organization (a business). It focuses on the value each step adds to the whole.
The time it takes to go through the process is the Lead Time.
Using this concept opens a perspective to look at processes in a business and relate them to (specific) values (outcomes), assign costs, commonalities, identify hold-ups (WIP) and so forth.
Identifying the (all?) Value Streams of an organization is the first step towards a DevOps Transformation.
Value Streams can be visualized in Value Stream Maps.