Steps per unit is how many steps to move one unit, probably inches in your case. With a 10 tpi screw, 200 step/rev motor and 1/2 stepping, you get 10 x 200 x 2 = 4000 steps per inch, for example.
The motor tuning settings should have no effect on torque or rpm, other than limiting rpm with the velocity. The Velocity setting is basically a speed limit. It's the fastest the motor will move, and is used during rapid (G00) moves. The accceleration is how fast you want to accelerate. If you try to accelerate faster than the motor is capable of, it will lose steps. Also keep in mind that stepper motor torque falls off rapidly as rpm's increase.