Alright I will try to explain with a simple way so you can understand , first you should understand and you should know that members are ranked you are a Newbie and I'am a Senior member as you can see .



As you can see here , Newbies aren't allowed to do some stuff and have max characters on their signature , and this rise with each Rank-up .


Signature compaigns basically gives you a BBCode to put in your signature that's redirect to their service/website , and logically they don't let Newbies enroll because newbies can't put much stuff in their signatures , however Senior members , Hero members etc ... signatures are so visible and people can see it and that's why they pay them better then others .

Please see here the ful llist of the signature compaigns and what ranks they accepts and the payouts aswell : https://bitcointalk.org/index.php?topic=615953.0

Most of them pay per post , let's say a signature compaigns pays 0.001 per post weekly , if you do 100 posts you will end up with 0.1 BTC / per week which is pretty decent since it's free and not too much efforts . Just make sure to not spam otherwise you will get banned