Base 8 (octal) math uses digits 0-7 instead of digits 0-9. The place values of the digits are based on powers of 8, rather than powers of 10. Example 456 (base 8) = 4*8^2 + 5*8^1 + 6*8^0 = 302 (base 10) Base 8 representation of numbers is convenient for expressing addresses and/or data for some computers, particularly those with word lengths that are multiples of 3 bits (12- and 24-bit computers, for example). The biggest reasons for its convenience are - only numeric digits are required (unlike hexadecimal, which uses 0-9 and A-F) - it is readily translated to and from binary. Each octal digit represents 3 binary bits.
