On the timeline, a solid dot appears in the frame, indicating that it's a keyframe. You'll also notice that the dashed line turns solid with an arrow at the end, telling you that the motion tween is complete. At this point, if you preview the movie, you'll find that the UFO hasn't actually gone anywhere. To make it take flight, you have to alter the UFO's position in the second keyframe. Simply grab the UFO in the last frame and drag it to the new position.
Flash redraws all of the in-between frames.
Now when you play the movie, the UFO streaks diagonally across the screen. Although the UFO now moves in the right direction, the animation still looks very flat. To spice things up, you can make the UFO seem to zoom past the viewer and recede into the distance. In a motion tween, you're not limited to moving an object across the screen. You can tween size, rotation, and color as well.