It is the center to center distance of between the metals having Minimum width and minimum spacing.
The minimum width & min spacing could be different.
it is usually decided by the manufacturing fabs cabality to make the accurate geometries with out
errors / issues.
metal pitch is the centre to centre distance of the same metal(for Eg, Metal1). Usually the width is taken as the minimum width of the metal(according to the DRC) so that the area can be kept min. The rails width is calculated by the power rating.