Computer programmers work in a number of different number-bases (for example: binary; 2, octal; 8, decimal; 10 and hexadecimal; 16). So to avoid confusion, they often indicate which base they are working in by prefixing their numbers with the first three letters of the base name (bin, oct, dec, hex).
The decimal number 25 is 31 in octal, so Oct 31 = Dec 25.
Placement of values in decimal are (e.g.) 1000 100 10 1
Placement of values in octal are (e.g.) 512 64 8 1
Thus 31 in octal is (3x8 + 1), or 25 in decimal (2x10 + 5).