Time for action – working with a Bézier curve
Bézier curves are very powerful. Blender uses them for modeling as well as for the F-Curves that control animation. Here is how to control them:
In Graph Editor, the current frame indicator is the vertical line with a box at the bottom. Move it left and right to scrub along the timeline of the Graph Editor. Watch the motion of the cube in 3D View.
Now, select the left dot on the upper Bézier handle, with the RMB, and move it up as shown in the next screenshot.
Press the LMB to release the handle. The curve will go upwards. Move the current frame indicator back and forth again and watch the motion of the cube. The Bézier handles can be moved with all the same controls as keyframes and objects.
With the cursor over either the Graph Editor or 3D View, press Alt + A to play the animation and Esc to stop.
What just happened?
You scrubbed the current frame indicator along the timeline of Graph Editor to see that the 3D views reflect how you are controlling...