Raw data is data as it's stored in the Zabbix database, with minor, if any, conversion performed. Retrieving such data is mostly useful for analysis in other applications.
In some situations, it might be a simple need to quickly graph some data together with another data that is not monitored by Zabbix (yet you plan to add it soon, of course), in which case a quick hack job of spreadsheet magic might be the solution. The easiest way to get data to be used outside of the frontend is actually the frontend itself.
Let's find out how we can easily get historical data for some item. Go to Monitoring | Latest data and select A test host from the Hosts filter field, and then click on Filter. Click on Graph next to CPU load. That gives us the standard Zabbix graph. That wasn't what we wanted, now, was it? But this interface allows us to access raw data easily using the dropdown in the top-right corner—choose Values in there.
If the item has stopped collecting...