FCP Editing Basics
Double clicking any clip in the Browser will open it in the Viewer. In the Viewer you can fine tune in and out points by pressing "i" and "o". Click and drag the image from the Viewer to the Canvas and an overlay window shows up. "Insert" and "Overlay are the two choices that you will be using primarily. Insert places the media that is in between your designated in and out points into the timeline beginning where the yellow playhead is and it pushes anything that is in the way to the right. Overwrite does the same thing except that it overwrites any media that is in the way.
The Viewer is associated with the active clip (whichever one has been double clicked) and the Canvas is associated with the timeline and more specifically with wherever the timeline playhead is.