When and how to use Inertia Relief

Hello everyone,

I've been looking for Inertia Relief information because I have a feeling I need this for an aeromodel I'm designing.

Google Searches haven't been fruitful, so I came here to try and get some info.

The first thing I'd like to ask is: What is Inertia Relief and why do flying airplane simulations use them? How hard is it to get to know Inertia Relief?

My second question is: Where can I get some material to start looking into this matter? 

Thanks in advance!