Fixed point math is a quick-and-dirty way to speed up any program that uses floating point. You define what precision you'll be using, i.e. 16.16 (16 digits on the right side of the decimal and 16 on the left) Essentially, you take a floating point number, and shift the decimal point over to the left until you're left with an integer.

This is usually done with #defines:

#define INT_TO_FIXED(x)      ((x) << 16)
#define DOUBLE_TO_FIXED(x)   ((long)((x) * 65536.0 + 0.5))
#define FIXED_TO_INT(x)      ((x) >> 16)
#define FIXED_TO_DOUBLE(x)   (((double)(x)) / 65536.0)
#define ROUND_FIXED_TO_INT(x (((x) + 0x8000) >> 16)

Fixed point math is/was primarily used in 3D graphics engines such as Doom, because working with integers is always faster than doing floating point, even with a FPU.

If anyone with a better knowledge of the subject would like to add more information, I'd appreciate it- my 3D graphics programming background is dubious at best.