It means that the persons have not accomplished as much of their project as they thought they would by a particular time. The words also can be used to mean that they have not travelled as far as they had planned by a particular time.
我們進度慢了！
本句 running late 可指實際旅遊的進度，趕不上原先的行程規劃；其引申義，則表示做任何事情的實際進度，落後於預期進度。