Acta Orthop Belg. 1991;57(4):399-405means that
The citation indicates that this article was published in 1991:
Journal: Acta Orthop Belg. (an abbreviation for "Acta orthopaedica Belgica")
Page number range: 399-405.
This is common format for scientific citations.
Obvious - not. What is obvious is that the scientific publishing community might as well be a secret society. They already have obscure notations, now all they need is a secret handshake.