It means they have a little information and are starting to form a picture of what is going on but, given the small amount of information they have, at the moment it's only a little picture. If they get more information, they will be able to see the big picture.
Sometimes, when someone is explaining or describing something to you, and they say "Do you understand?" or "Do you see?" you might say "Yes, I get the picture".
Student or Learner