Complex sentence: Each x86 assembly instruction is represented by a mnemonic which, often combined with one or more operands, translates to one or more bytes called an opcode; the NOP instruction translates to 0x90, for instance and the HLT instruction translates to 0xF4.
Split into simpler sentences: Each x86 assembly instruction is represented by a mnemonic, which in turn directly translates to a series of bytes which represent that instruction, called an opcode. For example, the NOP instruction translates to 0x90 and the HLT instruction translates to 0xF4.