Ah the magnetic timeline, the cause of and solution to all of life's problems.
Well that's a little overstated, but when you're finding your way through your first edit in FCPX the magnetic timeline can make things a little confusing, especially if you're coming from another video editing software.
That's where the Position tool (P) in Final Cut Pro (no longer strictly X) comes in handy.
As one of the commenters on this excellent tutorial from Ripple Training says, a helpful way to remember what to tool to use is:
P for "pretend like it's FCP7"