What does alignment to 16-byte boundary mean in x86

Question

Intel\'s official optimization guide has a chapter on converting from MMX commands to SSE where they state the fallowing statment:

Computation instruc

Answer 1

I'm guessing here, but could it be that "may not be aligned to a 16-byte boundary" means that this memory location has been aligned to a smaller value (4 or 8 bytes) before for some other purposes and now to execute SSE instructions on this memory you need to load it into a register explicitly?