everything is a number to a processor (well actually true or false, open or closed) an opcode is an instruction that the processor understands. But we really don't use opcodes, we use mnumonics (groups of letters that a human can easily remember). It is much easier to remember CALL then the opcodes for call: 0FAB, OFBA, E8, FF.
It is unfortunate that INTEL and AMD don't print the manuals anymore, I got them free about 10 years ago... You can get the PDFs though... The AMD explains what an opcode is better then INTEL.
"An opcode specifies the operation that the instruction performs."
Somewhere on my bookshelf, gathering dust, is an original Intel x86 manual. Back then, they didn't have the 32-bit and 64-bit instructions. 16-bit only... :-) Anyway, it was compliments of my college buddy, Bruce Ravenel, who was one of the principal architects of the x86 processor at Intel back in the 70's.