That's where you read the logs and select (on the top right) which chat to show.
The dropdown that lets you select which general chat to read. There is also one for misc. (achievements, item loot, system messages) and custom channels (the ones you manually join).
For each log, you can configure which messages should be captured in it. For example, you may want to capture your current group messages in the raid log as well.
Lets you copy logs in their raw format. Colors are displayed in the window for readability, but they are not preserved when copy/pasting.
You can also output the logs in a BBCode format, so you can paste that directly on forums. No need to manually type chat or item colors anymore!