Just about everything Understand knows about code is shown in the Information Browser (IB). The IB is used for all types of entities and it shows different things depending on the type of entity selected. You can quickly see information about all of its sub entities, what it uses and calls, some basic metrics, and it even lets you see everywhere that an entity is used, called, modified or interacted with. Information that is hierarchical in nature (such as a call relationship) can be expanded multiple levels.
The Information Browser is always just two clicks away, right click on any entity, even when browsing the source, and select View Information. By default the IB is pinned to the bottom left of the screen. Below is a sample Info Browser window for a C function: