It's a part of their skull.
http://www.skullsite.co.uk/Dog/dog.htm
'Many mammals, such as the dog, have a sagittal crest down the centre of the skull; this provides an extra attachment site for the temporal muscles, which close the jaws.'
http://www.britannica.com/EBchecked/topic/516268/sagittal-crest
'A sagittal crest is a ridge of bone running lengthwise along the midline of the top of the skull (at the sagittal suture) of many mammalian and reptilian skulls, among others.
The presence of this ridge of bone indicates that there are exceptionally strong jaw muscles. The sagittal crest serves primarily for attachment of the temporalis muscle, which is one of the main chewing muscles. Development of the sagittal crest is thought to be connected to the development of this muscle. A sagittal crest usually develops during the childhood of an animal in conjunction with the growth of the temporalis muscle, as a result of convergence and gradual heightening of the temporal lines.
A sagittal crest tends to be present on the skulls of adult animals that rely on powerful biting and clenching of their teeth, usually as a part of their hunting strategy. Skulls of some dinosaur species, including tyrannosaurs, possessed well developed sagittal crests. Among mammals, dogs, cats, lions, and many other carnivores have sagittal crests, as do some leaf eaters, including tapirs and some apes.'
https://en.wikipedia.org/wiki/Sagittal_crest
Some people call the prominent knob a 'knowledge knot'.
http://www.bordercollie.org/boards/index.php?showtopic=31945
