What does this mean in the Rails installation command?

In the Rails guides, it shows thee following for installing Rails:

gem install rails --pre

What is "--pre"?

Thanks.

abbreviation for “pre-release”. is a version to be installed

Luciano Sousa wrote:

abbreviation for "pre-release". is a version to be installed

Thanks. Can I specify a specific version.

For example
gem install rails --version 2.3.5
(that is two '-' chars).

As for --pre, is it a specific version? Like for example when I used it
it installed Rails3 and I think it was saying also beta4 or something?

Don't know that one.

Colin