What does the @ symbol mean in R?

From the help file (you can see this with help(":::")):

The expression 'pkg::name' returns the value of the exported
     variable 'name' in package 'pkg' if the package has a name space.
     The expression 'pkg:::name' returns the value of the internal
     variable 'name' in package 'pkg' if the package has a name space.

In other words ::: is used to directly access a member of a package that is internal (i.e. not exported from the NAMESPACE).

See this related question: R: calling a function from a namespace.