Meterpreter help

I'm new to infosec and trying to cover as much ground as possible. I've recently looked into meterpreter and I just want some clarification on a few things..

A dll is just a library that multiple applications can use to produce certain features in an application.. What are these features though?

Meterpreter can inject dlls into memory? Does that mean that they are run as soon as they are injected?

What is the stager socket?

I thought I knew a lot already but I've only just realised how much there is to know in infosec...