Velocity is a Vector. A Vector is a quantity that has both Magnitude and Direction. On a 2D plane (like the Scratch stage), movement on the stage (represented here as a yellow line; the length of which represents the Speed of travel) can be thought of as having an X component (shown here as a blue line and commonly referred to as the X Velocity) and a Y component (shown here as a red line and commonly referred to as the Y Velocity). In this demo, the left/right arrow keys control the X Velocity. The up/down keys control the Y Velocity. That effects both the Magnitude and Direction of the Velocity Vector. Clicking on the stage changes the Direction of the Velocity, but not the Speed (as would happen due to a hypothetical zero-friction bounce). The X and Y components the Velocity are then adjusted.
