Like a rebellious teenager, your pelvis longs to be independent.
Everything in our body is connected.  One bone is connected to another at a joint.  They are all held together with muscles and then we also have fascia (or connective tissue) holding everything together.  This means that we should be able to move fluidly.  You move one part of your body and another is affected.  When we move our spine in a particular direction our neck and head should come along for the ride.  Our body can twist, bend, flex, spiral and all flow nicely together.  And we want to be able to create flowing movement throughout the body.  That’s a good thing.
But it’s also really important to control movement in very segmented parts of the body.  I have found this to be especially important with people who have chronic pain and injuries.  You want to make sure two parts of your body are not locked together.  So you should be able to move your arms without affecting your ribs.
You should also be able to move your pelvis without moving your ribs, legs, or neck.  Large movements of the pelvis will affect the other parts of your body, but you should be able to get some range of motion out of your pelvis without anything else moving.  And this is important for proper alignment and proper muscle engagement.
As a runner or a walker, it is really important to be able to separate leg and pelvic motion.  Your legs should be able to move independently from your pelvis and your pelvis should be able to move independently from your legs.
Watch this video for further explanation and a subtle exercise to try.