pattern scan is searching for code , by remplacing the code by its original form (hex bytes) and removing the bytes that might change (usually the registers and addresses)
for example, code like this:
mov ebx,eax
mov eax,[esp+08]
mul [esp+14]
add ebx,eax
mov eax,[esp+08]
gets translated to byte code which is like this : 8B D8 8B 44 24 08 F7 64 24 14 03 D8 8B 44 24 08
and then to something like this : 8B ?? 8B ?? ?? ?? F7 ?? ?? 14 03 ?? 8B 44 24 08