Hexadecimal is a way of counting which makes more sense that decimal when dealing with computers. In hexadecimal, there are 16 digits, instead of the 10 we are used to. So when counting in hexadecimal, you get: 1, 2, 3, 4, 5, 6, 7, 8, 9, A, B, C, D, E, F, 10, 11... The reason why this is interesting when dealing with computers relates to the way the binary counting system works, but I won't go into that here.
Hexadecimal numbers are represented by prefixing them with 0x. ie: 0x15 = 16 + 5, or 21 in decimal.
xev displays X11 events as they happen. Every mouse movement, every key press and release (even shift!) is reported to the user. If you type shift-a then you will see a sequence of events:
Thus you can see what X11 sends as an event when you press the Meta / Alt key, for example.