Hexadecimal : Counting in base-16 as opposed to the more commonly used base-10. This system has always been used in trackers, both because it was rather easy to code when the first trackers arrived in the late 80's, and programming resources were limited, but also because the number 16 and it's halves and doubles are much more common in music than the number 10.
Instead of 0123456789 we now have 0123456789ABCDEF to count with. Like this:
Some useful numbers to memorize:
254 - FF (this is "max" for a 2column hexnumber)
127 - 7F (this is "mid" for a 2column hexnumber)
65535 - FFFF (this is "max" for a 4column hexnumber)
32767 - 7FFF (this is "mid" for a 4column hexnumber)