Hexadecimal is a system of counting in base 16, not base 10 as we normally do. To count in hexadecimal, you go 00, 01, 02, 03, through to 09, 0A, 0B .. through to 0F, and then you go to 10, and so on. Early gengineers had to hex edit genomes.
The largest number in hexadecimal is FF, which represents 255.
See the wikipedia article for more information.