The activity tab shows all the activies in progress or previously done by Lidarr.
The queue page allows you to view what is currently in progress or "to be done", it is a practical "to-do list" for all Lidarr operations that can be manually incited by a user. Actions are signified with icons, and fall into failures, grabs, imports, and downloads requiring human input (yellow import symbol).
The history page shows all operations previously completed via Lidarr, such as imports, grabs, deletes, fails, and downloads requiring human input (yellow import symbol).
Blocklist is formerly known as 'Blacklist'
The blocklist tab shows all the attempted grabs that upon downloading, failed. This can also be done manually by a user, in the queue tab.