This panel gives you an overview of how many pages have been optimized by NitroPack.io.
You also see how long ago a cached page has been purged.
Clicking the large number gives you a breakdown of the number of cached pages for every page type - mobile, tablet, and desktop:
Invalidate Cache & Purge Cache
Both of these buttons are used to clear the optimized page cache for your whole store. There is a notable difference between Invalidate and Purge.
This marks all cached pages as "stale". The existing cached content will be served to customers without any performance delay. The new optimized content will show up with a delay, whenever it is available.
Forced delete of all cached pages. Cached content is not served on the next page refresh. The new content will be seen immediately but at the cost of a loading delay.
What to use?
Using any of these buttons will clear all of your cached content. This will result in higher usage of your Optimizations Quota. Try not to invalidate/purge unless necessary.
In most cases, you will not need to use these buttons. NitroPack automatically invalidates and purges, depending on your actions in the WordPress back end.
If you have done a non-critical change of your front-end content, you may use Invalidate to ensure the change will be eventually seen.
If you have done a change you want to show immediately to your customers, you should use Purge.