"Animate Mode"
Animate mode is on by default. When animate mode is on, values are inserted at the current frame. Turning animate mode off instead alters all existing keys to create the desired change. This is handy for positioning lights at time 5:12, but you don't want the light to animate from where it was positioned at time 0:00 to where you moved it at frame 5:12. Rather you want to modify the 0:00 keyframe so that the light just moves over at all frames from 0:00 to 5:12. It does this by altering the keyframe that was already set at 0:00.
The following examples shows the top down view of a camera. The camera is primarily aiming to the left and slowly translating to the right. As the camera translates to the right it also oscillates up and down but stay aiming primarily to the left. In the first example with "Animate Mode" on, I changed the frame to frame 7 which did not have any keyframes and rotated the camera to point upwards which created a keyframe. When played back the camera has now incorporated the key made on frame 7 and will briefly point upward and then back to primarily pointed to the left.
The second animation demonstrates the same procedure but this time with "Animate Mode" off. Instead of making a key frame at frame 7 it adjusted all other keyframes so that the camera will point exactly straight up as desired on frame 7. When the animation plays back notice that the camera primarily points in the up direction rather than the left, but all the motion is preserved, just aiming in a new direction. This process can be done to any property.
Click the image to watch the movie