Free job boards allow you to post a job opening at no cost. Typically this post will be included in search results or job feeds on the given boards, organized by posting date. A job that was published today will appear earlier in search results than a job that was posted a month ago.
Some free boards have a premium option. You can sponsor the job post to have it appear more frequently and earlier in search results. Most sponsored posts have a time limit (for example, 30 days) after which the post will revert to the free version.
Paid / premium boards require that you pay to post at all. These boards may have advanced networks or offer other features which is why you must pay to post.