I have this file app/assets/stylesheets/config.rb with the following content:
http_path = "/"
css_dir = "."
sass_dir = "."
images_dir = "img"
javascripts_dir = "js"
output_style = :compressed
relative_assets=true
line_comments = false
what is it for?
To answer your question "what is it for", it's configuration for the Compass scss compiler. The reference in the other answer is handy, but for up to date details without speculation check out the details of the Compass configuration file here:
http://compass-style.org/help/tutorials/configuration-reference/
I had originally posted to also provide an answer for where the file came from (if you didn't create it yourself. I answer this because it was 'magially' appearing in my Rails project, and a Google search for generation of config.rb in Rails ends up here. I first supsected sass-rails, but someone from that project confirmed they don't generate that file. Then I figured out that auto-generation of this file was being caused by Sublime Text with the LiveReload package. See the related github issue.
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