The hip is a 'ball and socket' joint.
Various problems can affect the baby's hip as it develops.
Sometimes, although the ball is in the socket it can slip in and out of place. This is was it meant by the hip being dislocatable. Sometimes although the hip is in the socket it is not deeply in place and we call this hip 'subluxated'. Finally in some children although the hip is in the right place the socket does not grow properly and is too shallow. If the hip socket is shallow this may allow the ball to move from the position it should occupy.