The program counter in the cpu contains the address of the next instruction to be fetched.

The instruction is represented by a binary number, called an opcode.

Data (called the operand), for the instruction to work on, is at the address following that at which the instruction is stored.
If the opcode is ADD and is stored at address 10011000, then the data, eg 10, will be stored at 10011001.

The following actions are taken during a fetch/execute cycle to fetch the opcode and operand.

Fetch opcode (read memory).
Increment program counter.
Fetch operand (read memory).
Increment program counter.
Execute instruction.