Jenkins is a build and automation tool. It’s used to monitor the execution of repeated jobs, such as building a software project or jobs run by cron.
Traditionally, the process of building and deploying software has been a bit more drawn out. Usually developers would write code and commit it to a repository. Then operations or change/release management would be responsible for building and deploying the software to different environments. Then a QA team would perform testing on to ensure the code is functioning as expected.
With Jenkins, the build, test, and deploy process can be automated.
In short, Jenkins is a big part of the DevOps movement, and has forced organizations to think of common tasks differently: