What is cosmic background radiation? Why is it significant?
The energy that exists in the void of space. It is significant because there is a lot of it.
The temperature of space is not absolute zero, it is almost 3 degrees K. That is a really really small amount of heat but it is not zero. Since the universe is mostly empty space, somewhere around 99.99999999999 (thats a lot of nines) 999999999999999999 %, the fact that the empty space is 3 degrees K means that there is more actual energy in the background of space than in the stars.
Apparently the fact that this energy exists is evidence toward the big bang theory, although I am pretty sure there are other ways to explain it's existence.