NES instructions are broken down into a series of smaller, simpler steps by the CPU. Each of these steps executes in a fixed amount of time - a cycle. For examp

NES instructions are broken down into a series of smaller, simpler steps by the CPU. Each of these steps executes in a fixed amount of time - a cycle. For example, the "load value from memory into register A" instruction might take two steps: 1) fetch memory on the bus 2) move the fetched value into the register. Cycle-accurate means the emulator is emulating all of the little steps that make up an individual instruction. Instruction-accurate emulators ignore the smaller steps and treat instructions as indivisible. Cycle-accurate NES emulators only really matter for emulating certain graphical and sound effects.