Your campaigns can be rejected with the following message saying: "Website has no traffic history, we only allow sites with a certain traffic level".
When this happens your campaign has been rejected because we are unsure about it being a legit advertising campaign. This can be because the campaign doesn't look like it would be able to make a profit based on the look and feel of the campaign your trying to create, and what you want to pay per click. Because of todays high amount of malwaretizers trying to create fake campaigns that redirect to malware, we are very strict about letting in new advdertisers who we have no earlier record with.
So what can you do to get your campaign approved?
It will help if you provide us with some info on what you will be promoting, and how you see it being profitable for you. Perhaps showing us some stats for previous campaigns you have ran at other networks, and how they worked, or something else that helps convince us that your advertising campaign is what it looks like.