If you're talking about some class template named Vector that you have created yourself or gotten from a third party library then it is difficult to say for certain without knowing how it's defined but my guess is that it represents a mathematical/physics vector and 2 probably means you want it to be in two dimensions (i.e. contains two numbers).
Thanks for your comments, Yes it is part of openLb, it is hard for me to understand the code since i am new to C++ and the openLb has a lot of library. Do you have any recommendation?
Learn C++ with the standard library first.
Understand the Lattice-Bolzmann method (yuk! there are better methods for CFD!) second.
Then play around with the L-B examples (making sure that you can actually compile them).
I believe that
Vector<T,2> origin;
will, storage-wise, behave like
std::vector<T> origin(2);
However, this OLB version probably has quite a lot of convenient geometry-associated methods defined for it, making it behave like a mathematical/geometric vector.