Lead Time in software development is important because it helps you understand how fast work is moving through your software delivery pipeline. Together with other metrics that tell you about volume, engagement, quality and impact, you can understand if you are improving as a team.

In Athenian, it is calculated as the average time for pull requests to go from their initial commit till they are released. 

It is important to emphasize that Lead Time is calculated on released PRs only, if you want to understand what is happening to PRs currently still in the process, Cycle Time is a better metric. 

The concept of lead time comes originally from supply chain management and manufacturing. In these contexts, it can be defined as "total time it takes from receiving an order to delivering an item" or "the average length of time it will take a new set of inputs to move all the way through the operation".

In manufacturing, we've gotten very good at understanding how complex products with lots of dependencies, for example, a car, can be built in the most efficient way. Building software however is not a manufacturing line and a unit of measure isn't as clear (we all know that lines of code are a terrible metric that can not be compared), therefore we can take inspiration from manufacturing concepts but need to evaluate and apply them to our own domain. 

The popular book Accelerate, talks about the Lead time for changes and defines it as "the time it takes to go from code committed to code successfully running in production."