In animation, keyframes are the beginning and end of a transition. This transition could be movement, rotation, or scaling of an object. It can also be a change in the color of an object, the brightness of the light, or almost anything that you can set in Blender. The animation is a sequence of keyframes. So let's start by making a simple animation:
If the cube is not selected, select it with theRMB.
With the cursor over the left-hand 3D View window, press the letter I key, (I for Insert), to insert a keyframe. A drop-down menu will appear as shown here. Select Location and press theLMB.
In the Timeline window at the bottom of the Blender window, drag the current frame indicator right until it is over the 20. As you do that, the current frame text fields in the lower left of the 3D Views will change, as will the current frame button in the Timeline window.
With the cube selected, grab the green 3D manipulator in the left-hand 3D View...