Capistrano (in a previous life SwitchTower) is a gem created by Jamis Buck that allows for deployment, monitoring and capistrano gateway connectivity implementation for your rails applications to their respective web and database servers.
Hands down there is nothing on the open source market for Ruby on Rails that meets the rich feature set and cabaility that Capistrano displays.
Deployment can mean many different things in software development and in the case of capistrano, the common meaning is that of taking your project or a piece of an application from development into production.
In capistrano terms, this can mean setting up the entire servers environment and configurations, attaching to the webserver setup and tying in to a database. And as I mentioned before, you can even use it to monitor operations.
For any lead developer of a rails project, learning capistrano and how to use it for deployment will ultimately save you time and make you look cool when talking to your peers at Starbucks about your latest deployment strategies.