In operational terms nothing happens, it's business as usual.
Your application will still be able to send messages through Realtime but you'll be billed with an overage usage of $1 per each million messages.
I'll give you an example:
Let's say you are using the $30 USD basic Ignite plan.
If in a given month you send 40 million messages your billing for that month will be:
$30 USD + 9 million messages in overage * $1 USD = $39 USD