DARTH VADER wrote:
What exactly is space time ?

A four dimensional metric space of indefinite signature.

Why cant we travell through the 4th dimention (time axis)?

We can and we do. The rotating second hand on my watch is a clear
indication of this. Time on this level seems to have a directional
arrow associated with it, but that is a thermodynamic consequence
corresponding to increasing entropy. On a fundamental level, most laws
of physics don't care which direction time goes.