When I don't understand a concept, I go back to the last milestone and try to understand it.
After doing that a few times if I am still not clear with it then I jump to the very beginning and try to grasp it(I do this because chances are that I may not have understood a few concept along the line).
By this point, I still may not understand the concept but I would definitely know when can I understand it(i.e. the prerequisites to understand this) so I move forward and when the time is correct I would jump back to this topic.