You have signature space on your ID, and signature campaign owner rent your signature space to put their advertising on it.
They have rules and requirements, you have to follow that if you want get payment.
The requirements usually are Full member level or above, have a good post history and etc.
The rules usually make particular number of post on some period of time, like 30 posts on bi-weekly period, called fixed post campaign.
Or they pay you based on number of your post, called pay per post campaign.
And usually they restrict you to post on some boards / section, or have minimum character per post.
Be careful, your post must be constructive to get payment from them