A menu presents a list of options to a caller to choose from. For example, "press 1 for sales, press 2 for accounts etc...". You can then choose how to route the call based on which options the caller presses.
When you first visit the Menus page, you'll see a list of previously configured menus in your account:
Each menu will show the name, a list of configured options, and a status. Menus with an Active state have received calls within the last month, Quiet if no calls have been received for at least 1 month, Inactive with no calls in the last 3 months and Unused if no calls have ever reached the menu.