What is free dining?

[gabbyfamily]

I am in the midst of planning our first family Disney trip. We are staying four days at Disney, then going on a 6 day cruise. It is so overwhelming planning this trip! I have read on here about free dining. What is this? We are planning on getting the disney dining plan - is there some times where they offer this free? We are going to Disney World January 26-30.
Thanks!
Carey