The assumption is that you have read through the prereading, click here.
To move at a constant velocity means that the speed remains unchanged and the acceleration is also zero. It is constantly zero. This motion is called "constant velocity."
The variable Δx is the displacement. Displacement is the change in position. Therefore the displacement is the final position minus the initial position. Mathematically it is written as Δx = x  x_{O} where
Δx = the displacement (change in position)
x = final position
x_{O} = initial position 
Usually for problem solving the initial position, x_{O}, is equal to zero. This will make some of our problem solving easier.
There four types of velocities to be considered:
Δv = the change in velocity
v_{O} = initial velocity
v = final velocity
v_{AVG} = average velocity

Similar to displacement, Δv = v  v_{O}. Any time the change in a quantity is mentioned it is always measured as the final amount minus the initial amount. The final amount occurs later in time.
Usually when people think of velocity, they think of moving fast or slow. But velocity can also be though of as how quickly you change position. Velocity times time is the displacement –or change in position.
Here is how to visualize a simple constant velocity problem.
