Signature ad campaigns are a series of advertisement messages that use forum users' signature space to share a single idea and theme which make up a marketing communication. Wearing a signature advert allow users to earn some BTC from their constructive post. However, users should know forum rules and keep in mind that inconsistent and spam posts are not tolerated here. Also wearing a paid signature may give you a presumption of guilt when you would be found violating forum rules.
You can partecipate to a campaign provided such campaign is still active and accept users of your rank/activity.

Here an overview of the current signature advertising campaigns: https://bitcointalk.org/index.php?topic=615953.0