in application of gauss law what does infinte length of a rod mean ?

All Gauss' Law implies is that the NET electric flux through a closed surface containing NO charge must be zero. It says NOTHING about the magnitude or direction of the electric flux through any particular point on the closed surface. If the electric field is exactly zero everywhere on a closed surface there can be no NET charge contained within that surface.

 

As you make the linear charge longer as it approaches infinity the surrounding Gaussian surface also approaches infinity. These two effects cancel out!

 

Infinite length of the road means parallel to the surface.