New turfed garden lawn.

[trt]

Had my back garden re-turfed 5 weeks ago, the guy who did the work said dont walk on it for 3 months. What harm what it do if I do?

[buildersmate]

It'll squash it down with footmarks and hence not look flat any more (because the turf isn't rooted into the subsoil). It'll compact the soil and tend to kill the grass where this has happened.
Lay a plank across it if you must walk over it.