Real time bidding is how display advertising is beginning to be bought across websites for the most part. there are many different companies who have a platform and ad server that support real-time bidding to deliver yours or your clients advertising messages to the right target audience.
So what actually IS it
Basically, real-time bidding means that you can change things on the fly, based on successes and failures of your campaign. It kind of reminds me of the stock market, but less risk and fluctuation. Advertising pricing goes up and down across websites depending on supply and demand factors, amongst other things such as how narrow or broad your target scope is. When you place a bid amount for advertising, you are telling the ad server how much you are willing to spend for impressions or clicks.
It kind of reminds me of the stock market a little bit, but less risky and less fluctuation.
Its pretty great also to be able to tell which of your creatives or ad sizes are performing better than others. This way you can alter the campaign as you go.