I assume it means 150% of the speed of light.

Probably it means “really fast”

In RPG terms, travel time is subject to a “hyperdrive factor”, which is multiplied into the overall travel time between two systems on a jump.

In this context, the .5 meant that the Falcon’s hyperdrive halved travel time vs the best standard military grade hyperdrives of the day, which were x1 units.

Doesn’t mean anything, just something George wrote to sound cool. Same thing with the Kessel and parsecs, which only were only retroactively made to make sense later.