"Show non-Google ads from another URL" ?

[Tension]

Very often I get the blue box on some of my sites because Adsense has nothing relevant to display at the moment. I chose the blue box so I could more easily see if something was wrong but I have no interest in public service ads either.

That leaves one alternative: Show non-Google ads from another URL.

Exactly what do they mean by this, how do I set up another ad and what url should I then paste in?

 

[Brian]

It could be another ad network you use, or simply your own ad, I use the empty ads to show an ad for another site of mine

 

[Tension]

Ok thanks, is there anything I have to be aware of here? I mean, if there is ads from some companies that I cannot put up on the same site as Google's. I heard something about Yahoo for example.