Standard Work is one of the bread and butter concepts of Lean. Basically, Standard Work is a very specialized form of standardization that balances a precise sequence of work for a single operator to a takt time. And that sequence, by default, also defines the amount of standard work-in-process that will be present at any given time.
Effective Standard Work has a tremendous impact on quality, delivery, cost, safety, and morale, making it one of the broadest reaching tool in the Lean toolkit. It also acts as an amplifier for many of the other tools and principles. Kanban, andon, flow, poka yokes, and many others are all made more effective when they are building off the stable platform that Standard Work provides.
Standard Work also powers the plan-do-check-act cycle. Taiichi Ohno once said that "Without standards, there can be no kaizen." Standard Work prevents the backsliding that prevents continuous progress with the improvement cycle.