Google Display Network (GDN) allows your advertisement to be shown on relevant content website only if it is within your marketing budget.
Example: If you have a $1000 marketing budget per month, this equates to $33.33 per day. If your targeted keyword cost $1 per click, your budget can only last for 33 clicks. After 33 potential customers have clicked on your advertisement, your advertisement will not be shown anymore until the next day.