Non-visible functions are asterisked

Dear R-Helpers,

I suspect I'm about to ask a FAQ, but I haven't been able to find an
answer in the FAQ, AItR or an R Site Search. When I look at the methods
of summary (below) it says, "Non-visible functions are asterisked". I
looked at the help file for summary.princomp, which did not comment on
it being non-visible. I ran its help file example, which printed visible
output. I did not notice how it differed from other functions, like
summary.data.frame that is not marked non-invisible. What does the
non-visible mean?

Thanks,
Bob 

methods(summary)

 [1] summary.aov            summary.aovlist       
 [3] summary.connection     summary.data.frame    
 [5] summary.Date           summary.default       
 [7] summary.ecdf*          summary.factor        
 [9] summary.glm            summary.infl          
[11] summary.lm             summary.loess*        
[13] summary.manova         summary.matrix        
[15] summary.mlm            summary.nls*          
[17] summary.packageStatus* summary.POSIXct       
[19] summary.POSIXlt        summary.ppr*          
[21] summary.prcomp*        summary.princomp*     
[23] summary.stepfun        summary.stl*          
[25] summary.table          summary.tukeysmooth*  

   Non-visible functions are asterisked