Short answer: Hourly billing charges for a VPS per hour of existence, with a monthly cap at the plan's monthly price (reached after about 730 hours). It makes short-lived servers, tests and CI jobs cost cents.
Hourly billing explained
Cloud providers bill per hour or per second and charge for a server as long as it exists, even when powered off; to stop paying you delete it, optionally after taking a snapshot. Traditional hosts bill monthly or yearly in advance.
See it in the data
Frequently asked questions
Does a stopped VPS cost money?
At most hourly providers, yes: compute is reserved until the server is deleted.