Hello Sam,

The following goes over it in detail: 

https://documentation.commvault.com/2023e/expert/optimize_storage_space_using_deduplication.html

But for the short answer, Primary records point to a written chunk of data on your disk. 

Secondary records point to a Primary record. One Primary can have multiple secondary pointing to it and this is the % savings. Each of those secondary records are an attempt to write data that was mitigated as it was already on the disk. 

Kind regards

Albert Williams