Original source: Wikipedia - Roman numerals
A bracket multiplies the value of a group of letters by one thousand, similar to a macron (vinculum). Therefore, [V] represents five thousand, [X]M represents eleven thousand, [[X]] represents ten million, and so on.
You can also use the "subscript" next to a bracket as well for powers of one thousand. For example, [M]^V is equal to 1000*1000^5 = 1000^6 = 10^18 = one quintillion.
Examples:
4,096 = M[V]XCVI
65,536 = [LXV]DXXXVI
1,048,576 = [MXLV]MMMDLXXVI
16,777,216 = [[XV]MDCCLXXV]MMCCXVI
4,294,967,296 = [[M[V]CCXC]M[V]CMLXV]MMCCXCVI
18,446,744,073,709,551,616 [[[[[[XV]MMMCDXLV]MDCCXL]M[V]LXX]MMMDCC]M[C]DL]MDCXVI