Where each digit in a number is multiplied by its place value, and the place value is larger by "base" times for each position going left.
Our decimal number system uses positional notation, and so does binary, octal, hexadecimal etc.
Example: In "base 10" (the decimal number system):
• For 327 the 3 contributes 3×10×10, the 2 contributes 2×10, the 7 contributes 7
Example: In "base 8" (the octal number system):
• For 327 the 3 contributes 3×8×8, the 2 contributes 2×8, the 7 contributes 7
(Note: by contrast Roman Numerals do not use positional notation: X always means 10, or minus 10 when in front of a bigger digit.)