Han Solo talks about making the "jump to light speed." If the Millenium Falcon is somehow jumping to light speed, it implies a nearly instantaneous acceleration. The Falcon might be traveling along at 50 miles per hour, and then suddenly it's traveling at 186,000 miles per second. Let BMW try to beat that acceleration! It's no problem for Han to accelerate the Falcon from zero to 60 miles per hour in five seconds. Inertia will push him slightly back in his seat. But accelerating from zero to 186,000 miles per second in five seconds will push Han back so forcefully that he'll become a splat on that fine vinyl upholstery.
The speed of light is so fast, that to accelerate to it safely would take months! We measure acceleration in g's, with one g equal to the acceleration caused by Earth's gravity—the acceleration of falling objects on Earth. The reason we measure acceleration in terms of gravity is because the two have the same effect. The gravitational force on an object is equivalent to the inertial force on an object undergoing a comparable acceleration. Just as gravity pushes you down against the Earth, inertia pushes you back against your seat.
We experience higher or lower g forces when we are rapidly changing speeds or directions. Normal humans can withstand no more than 9 g's, and even that for only a few seconds. When undergoing an acceleration of 9 g's, your body feels nine times heavier than usual, blood rushes to the feet, and the heart can't pump hard enough to bring this heavier blood to the brain. Your vision narrows to a tunnel, then goes black. If the acceleration doesn't decrease, you will pass out and finally die. The Air Force's F-16 can produce more g's than the human body can survive. We're forced to limit the acceleration of planes and spacecraft to a level humans can survive.
If we need to accelerate for extended periods, the level we can withstand is even lower. We can withstand 5 g's for only two minutes, 3 g's for only an hour. For the sake of argument, though, let's try to tough it out at 3 g's for a little longer. For Han to take off from Mos Eisley and accelerate at 3 g's to half the speed of light would take him two and a half months—hardly the makings of an exciting movie. Even at 9 g's, it would take him nineteen days to reach half the speed of light, though he'd be dead long before the ship reached that speed.
Since Star Wars ships are constantly undergoing rapid accelerations and decelerations, they must have found some way to solve this problem. Perhaps they have learned to manipulate inertia. Just eliminating it for a fraction of a second could allow a rapid, effortless acceleration, after which point inertia could return and the Falcon could cruise at a constant, high velocity.
Of course, the force that makes us stumble back as the subway car accelerates doesn't seem completely conquered on the Falcon. In The Empire Strikes Back, the Falcon's jump to hyperspace throws Artoo across the deck and into the open engine pit. Perhaps some of Han's "special modifications" need a tune-up.