One of the core principles of Lean is to create flow. It is impossible to achieve with long setup times.
When it takes an extensive amount of time to switch from one product to another, operators must run large lots to produce enough parts to keep production flowing.
With single minute exchange of die, though, the lots can be much smaller. Single minute exchange of die, or SMED, is exactly what it sounds like. It is the ability to remove one set of die from a machine and install another set in a very short time. While purists tend to shoot for one minute, most people find that exchanging die with only a single digit (i.e. 9 minutes or less) is sufficient to make marked improvement.
Single minute exchange of die makes extensive use of standardization, 5S, and visual controls to accomplish. Another main component is to convert internal setups (actions that must be done when the machine is shut down) to external setups that can be done while the machine is processing parts.