The sole purpose of this namespace is to be able to import all functions and symbols that are expected to be used within operator() for indexing and slicing. If you already imported the whole Eigen namespace:

using namespace Eigen; 

then you are already all set. Otherwise, if you don't want/cannot import the whole Eigen namespace, the following line:

using namespace Eigen::indexing; 

is equivalent to:

using Eigen::all;
using Eigen::seq;
using Eigen::seqN;
using Eigen::lastN; // c++11 only
using Eigen::last;
using Eigen::lastp1;
using Eigen::fix;