When we talk about data in Matomo we often distinguish between RAW data, and report data. (You might also hear report data called aggregated data or archived data.)
RAW data (unprocessed data)
RAW data is the tracking information gathered from your site and stored in your Matomo database. Each time a visitor visits your site, tracking information is sent to Matomo. The tracking request sent to Matomo looks like this:
This HTTPS request contains all the relevant information about your visitor like the page URL, page title, browser type, device, visit time plus much more depending on the request. Learn more in FAQ: what data does Matomo track?
This data is split up and then stored in your Matomo database. As you can imagine with thousands or millions of visits this can consume a lot of space. The data is then archived and aggregated into report data. After this has happened the RAW data is usually redundant as the relevant information is now in the report data. However the RAW data is still used when you create segments of your users. You can configure Matomo to delete old RAW data (unprocessed data).
The simplest description is you can consider the RAW data the the raw material of a cake, flour, eggs sugar etc.
Report data (archived data)
Report data (the cake) is derived from your RAW data. Once the reports are created and the RAW data has been removed it is not possible to deconstruct the report data to create RAW data. However, Matomo can keep the report data forever. RAW data could potentially be kept forever also but due to the amount of space this would require it is not entirely practical and generally not required.
Report data can be found in the reports in the Matomo left hand navigation menu (most left menu links will show report data).
You can configure Matomo to delete old Report data (archived data).
What issues arise if the RAW data is not available?
Please see the section in this FAQ called: What is the impact of deleting historical RAW data?