You can find the answers in FAQ in the Community. I tried to copy and paste but it was not working. If still unable to find this, please email firstname.lastname@example.org and they can help for sure.In short, rumbling is when a video has been analyzed by their software and requires some manual inspection to determine if it will be suitable for profit sharing status.
This should answer your question. Hopefully it helps to understand.
You can find more answers here: https://community.rumble.com/threads/new-feature-added-x-rumbles.10086/page-2
Chris Rumble says in this thread: "Rumbles" will become a very important part of video metrics in 2019. This is our first rollout to help our loyal users get a head start in building their authority. I would highly suggest commenting, upvoting and downvoting comments you like and don't like.