When you’re setting up a campaign, you will notice that you’re (sometimes) asked to set an “hourly budget”, “daily budget”, or “hourly/daily spend pace”. This is simply the rate at which you would like your campaign to spend credits.
In other words, if you set an hourly budget of 50 (in whatever display currency you choose for your credits) on your campaign, it will aim to spend that much (of credit) per hour where possible.