[Rails] rails asset cache

I have a of line code like
<%= stylesheet_link_tag "styles",:cache => "cache/styles" %>

And it produces source as

<link href="/stylesheets/styles.css?1279620123" media="screen"
rel="stylesheet" type="text/css" />

config.action_controller.perform_caching = true

My concern is what is 1279620123 after question mark(?) above. If such a
random number always appended, whether caching in browser happens or
not?Please give me a good idea

Thanks
Tom
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