What is paid to post you ask. There are companies out there that are hired by forums and blogs to bring traffic to their sites. Those companies then hire individuals to post on those sites and pay them per post. On average you make between .10 to .50 per post, depending on the job.

These companies expect nothing more then for you to provide quality posts for the companies who have paid for their services.

To make it as a good poster here are some things you will want to follow and keep in mind.
1) Do not make short-" yes/no" or "me too" types of posts. Keep them at the minimum between 15 and 25 words unless otherwise requested.
2) Never disclose that you work as a paid poster on any forum you are paid to boost.
3) Always respect the rules of the forums you are on.
4) Never Spam
5) Be careful when using affiliate links in your signature or posts - often it is not allowed by the company who hired you.
--Basically, just follow the rules!

Here are a couple of sites, that I guarantee are legit, to start you off:

Kickstart Your Forums

Forums First