These answers assume you know what a DSL is, or are familiar with Make or Ant. If that’s not the case, here’s a (perhaps grossly oversimplified answer):
Rake can be downloaded and included in ruby projects as a ruby gem.
Once installed, you define tasks in a file named “Rakefile” that you add to your project.
We call it a “build tool” because Rake comes with some libraries that make it easy to do tasks that are common during the build/deploy process, like file operations (creating, deleting, renaming, & moving files), publishing sites via FTP/SSH, and running tests.
For more information, here’s the project documentation: http://rake.rubyforge.org/