Choose this option to let Jenkins install this tool for you on demand.

If you check this option, you'll then be asked to configure a series of "installer"s for this tool, where each installer defines how Jenkins will try to install this tool.

For a platform-independent tool (such as Ant), configuring multiple installers for a single tool makes not much sense, but for a platform dependent tool, multiple installer configurations allow you to run a different set up script depending on the agent environment.