Polarisation in itp file

Dear Gromacs Users,
I am doing simulation using Gromacs software. I want to add polarisation in itp file. Here I found format of polarisation in itp file.
[ polarization ]
; Atom i j type alpha
1 2 1 0.001
But I am confused what is atom, i, j here and how they take. Can anyone help me regarding this.
Thank you very much in advanced for your kind help.

Hema Teherpuria