The Smart Folder allows you to access important messages from more than one account, in the same place.
It can display all unread, starred as well as recently sent or read messages. How recent these messages are is also configurable.
The intent of the Smart Folder is to display any e-mails that might still require your attention.
In other words, this is the list of messages that might require attention, or that you might need to come back to.
This folder is enabled by default, but can be disabled in Settings > Smart Folder > Enable Smart Folder.
The Smart Folder configuration can be made inaccessible (greyed out) if it is the app's startup screen with no alternative (the Account List). If the folder is greyed out but you need to access the settings, go to Settings > Look and feel > Disable the "Hide the account list screen" option.
To exclude the account from the Smart Folder.
Long press on the account -> options and folders -> folders.
Let it refresh.
Long press on the Inbox or tap the Settings icon (gear) on the right.
You'll see a popup window where you can turn off whether this folder is included in the Smart Folder.