Hexadecimal is a number system where there are 16 base numbers, so the counting system is 0-9-A-F. This is different from the decimal system (normal numbers) where there are 10 base numbers, 0-9. Because a hexadecimal number can be composed entirely of the digits 0-9 and thus mistaken for a decimal number, it is common to indicate a hexadecimal number by prefixing it with "0x". For example, 0x12345678.
Hexadecimal, or hex, numbers are frequently used in SimCity 4's internal game files. All TGIs are listed in hex, and hex numbers are also commonly used as values of various properties. Some editing programs automatically convert hexadecimal values to decimal for easier human understanding, while others do not.
See the wikipedia article on Hexadecimal