By default, your server will scale up after one hour of using more than you are paying for. Essentially this allows you to "burst" above your paid usage for free an hour each day.
After an hour of using more than you're paying for, your server will scale up and you'll pay for another unit of that resource (1 core, 1 GiB of RAM, or 10 GiB of storage).
Network usage is simply billed based on how much data your transfer out of our network. Inbound data is free.