Significance of the magic number of btree pages..?

Hi to All,
Iam new to the group and to postgresql. Currently Iam trying to understand how does the planner work by using gdb. During the execution of a plan I observed a variable "BTREE_MAGIC" which is defined 0x053162 and called, 'the magic number of btree pages'. I couldnt understand the concept behind the 'magic number' and the "0x053162". Can anyone kindly explain the significance of the 'magic number of btree pages' or refer to some document where I can get idea about this.

Thanks in Advance,
Anil