MMX, unofficially "
MultiMedia eXtensions" (although Intel won't admit that), is a set of extra opcodes to perform several operations very
fast, especially useful in graphics and sound processing. You can do stuff such as adding, subtracting and multiplying 8x8bytes at a time, with or without "
saturation" (so that 200 + 100 becomes 255 and doesn't "wrap around").
Of course, you cannot do
FPU stuff while you're doing MMX, or you will
die horrendously.