Its called a parameter pack and refers to zero or more template parameters:
http://en.cppreference.com/w/cpp/language/parameter_pack
std::size_t ...Rs
is the parameter pack of type std::size_t.
A variable of that type (e.g. Rs... my_var) can be unpacked with:
my_var...
This pattern is heavily used in forwarding an (unknown) amount of arguments:
template < typename... T >
Derived( T&&... args ) : Base( std::forward< T >( args )... )
{
}