Understanding Gets, Pins and Reloads - V$librarycache

Can someone please explain in detail the gets, pins and reloads in v$librarycache.

How are these columns related to each other? When would gets, pins and reloads increase?  

I read some definitions like

The GETS column indicates the number of times, since database instance startup, that the

System requests handles to library objects in this namespace.  

The PINS column indicates the number of times, since database instance startup, that the

System issues pin requests in order to access objects in the cache.  

Will both not go 1:1 ratio?

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