It appears someone is trying to promote clouds on the network. I've seen this ad on various sites, and as a sidebar and banner style ad.
What is it for, clicking it results in nothing but selecting a part of the ad, as can be seen in below screenshot. Which makes it clear the clouds are just made up of white circles. Is there some hidden message?
As far as I remember (from digging into it in the past), it's a generic banner used as result of internal error.
The answer by the (former) SO developer to the linked old bug report hints that as well:
The clouds are due to an ad tag being implemented improperly
"Implemented improperly" >>> Error >>> generic banner so there won't be an empty gap