what does `x` mean in PIC opcode format?
'x' means 'does not matter.' Those are the bits of the "CLRW" instruction.

When you type "CLRW" into an .asm file and assemble it, this opcode gets converted to '00010xxxxxxx' (12 bits, since this particular PIC architecture is 12-bit.) It doesn't matter what the x's are; if the PIC executes anything with the first bits as shown, it will be treated as a CLRW instruction.