Complex sentence: The CPU then fetches the instruction from main memory via the data bus into the memory from the MDR is then placed into the current instruction register (CIR), a circuit that holds the instruction temporarily so that it can be decoded and executed.
Split into simpler sentences: The CPU then fetches the instruction from main memory via the data bus into the memory data register (MDR). The value from the MDR is then placed into the current instruction register (CIR), a circuit that holds the instruction temporarily so that it can be decoded and executed.