Before you get to the Pyramid Challenge game you need to know a thing or two about hexadecimal numbers. Usually people use the number system known as the decimal system which you are quite used to working with. But there are other number systems, such as the hexadecimal system (not to mention the binary and octal number systems).
The hexademical system uses the same numbers as the decimal system plus six letters:
HEXADECIMAL DECIMAL 0 0 1 1 2 2 3 3 4 4 5 5 6 6 7 7 8 8 9 9 A 10 B 11 C 12 D 13 E 14 F 15