ReferenceTooltips allows you to roll over any inline citation* to see reference information, instead of having to jump away from the article text.
* in the article space or the Wikipedia: or Help: namespaces. See Talk page, comment dated 05:07, 29 October 2013 (UTC).
This gadget is enabled by default. However, if it is currently disabled for you, you can enable it at Preferences → Gadgets → ReferenceTooltips.