Simply put conversion credits is your currency when using Filestar. You load up on credits on our Pricing page and choose the package that suits you the best. Most conversions and skills cost 1 credit each. But the credits needed for a skill can vary depending on what you want to do. If you use very heavy files, many files at once, or more unusual skills, more credits could be needed. In the app, you can preview the credits needed for an operation before you go ahead.
You can see your credit account balance in the desktop application if you click on "About" in the menu.