Refreshing your job advertisement means extending its duration on the site.
Each refresh consumes a job from your subscription.

This implies that if you have subscribed to a pack of 5 jobs, you can either post 5 jobs or post one job and refresh it 4 times.
Also, if you have purchased a single job posting, you will not be able to refresh it.
Single postings purchased are a one time posting that cannot be refreshed or edited.