# What does id (cluster) mean in gee?

I wanted to use poisson glm model which would take serial correlation (AR-1) and overdispersion into account. I was directed to use gee, but gee (function of gee package in R) requires parameter id which I don't have an idea what that means. The documentation says it identifies the clusters but I don't have an idea what the cluster concept is about. Can you please explain the cluster concept of gee? Is it somehow related to the autocorrelation thing?

(source: Halekoh and Højsgaard, 2006 in JSS paper on geepack library)