what do they mean by "add insulation" when replacing the rear deck speakers

[austinDeville]

I have a 95 deVille and I am about to replace the rear deck speakers. In preparation i have read the posts about the topic and a number of people have recommended "insulating the rear deck" while you have it out. What do they mean? Where is the insulation supposed to go? What kind? Do I attach it or just lay it in somewhere?

thanks in advance. Maybe it will be obvious once I have it taken apart, but I'd rather be forewarned.