This command appears on topic pages. It marks the topic as "forgotten" for you. This means you won't see it again, even if others keep posting to it.
You can see topics that you've forgotten by going to the conference's page and doing a [Forgotten/Retired] command. This will include any forgotten topics in the topic list - they will have a "G" flag.
You can undo a forget by going to the topic's page and doing a [Remember] command.