Each plan gives a certain number of credits per month. Depending on the number of rows you extract in a task, the screenshots you capture, and whether a site is Premium, each task will cost you from one to several credits.
With each credit, you can extract 1 row of data from a page or capture a screenshot.
For example, if there are 50 products listed on a webpage and you need to extract them, it would take 5 credits. If you extract each product's details as well from its dedicated detail page, that would take an additional 50 credits.
If you monitor these 50 product detail pages for changes checking every 3 days, it would take about 50x(30/3) = 500 credits per month.
There is a small number of sites that require premium proxies or a large volume of files to load. Those sites are marked as Premium and each run on these sites has a minimum credit cost between 2 and 10.
When your billing cycle (year or month) ends, your credits will reset. Unused credits will also be reset and will not be carried over, unless before the billing cycle ends, the plan is upgraded in which case any unused credits will be added on top of the credits you receive from the upgrade for the duration of the plan to which you have upgraded.