Public randomization device
A sunspot is just a publicly observed random variable upon which players can condition their actions (formally it's part of the history of the game).

It's helpful because it is a nice easy way to convexify the set of equilibrium payoff vectors. In many papers (in my experience) it is a convenient assumption that simplifies things, yet may not be necessary--that is the effect of sunspots can be accomplished without their use, it's just more of a hassle.