View the contents of the selected folder or mailbox (i.e. go down a level in the hierarchy). For example, if the Records source is selected, you can use the search box to enter the name of the folder or mailbox that you want to view, select it in the record list, and then choose Contents of Record. This is often quicker than expanding the various folders inside the Records source to find the one you want.
If a message or non-mailbox file is selected, Contents of Record will instead say Reveal in Container, and it will select the containing mailbox or folder in the source list and select the message or file in the records list. This lets you search for a message or file in Records, a tag source, or a smart folder, and then reveal it in its original context.