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In large projects, your code might depend on many external packages. A manifest contain meta-information about what your software is doing and how to set it up. The documentation is quite exhaustive on the topic: wiki.ros.org/Manifest

Practically speaking, you can very well just "import roslib; import rospy" and your code will still work. You might however want to think of writing a manifest as soon as you start depending on other libraries and want to share you code with other people.