In the notation they are using, translation is the relationship between the coordinates used by me and the guy sat next to me. Uniform motion is the relationship between the coordinates used by me and someone sitting in a passing train.
In the first case, the difference between the coordinates is just a constant offset. If I say something is at ##x=-1## (i.e. 1m to my left), the other guy says it's at ##x'=-2## (i.e. 2m to his left). That is true for all times.
In the second case, the difference between the coordinates is a time-varying offset. If the train is moving at speed v past me, I say that the passenger is at ##x=vt## at time t. However the passenger regards herself as at rest at ##x''=0##, and me as moving at -v, so I am at ##x''=-vt##.
That's the difference. The first one leads to "the laws of physics are the same at all places" and the second to "the laws of physics are the same at all speeds".