gls procedure in nlme

I need help with gls{nlme}.
Specifically, I am estimating an equation with AR(1) using 
maximum-likelihood. I am not understanding the correlationoption below. 
Help appreciated.

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library(nlme)
eq1<-log(chnimp)~log(chempi)+log(gas)+log(rtwex)+befile6+
                  affile6+afdec6
reg1<-gls(eq1,data=mydata,correlation=corAR1(),method="ML",verbose=T)