MC, MR, and M+ are memory functions found on many calculators. MC stands for "Memory Clear" and is used to clear the calculator's memory of any stored values. MR stands for "Memory Recall" and is used to recall the value that is currently stored in the calculator's memory. M+ stands for "Memory Plus" and is used to add the current displayed value to the value that is already stored in the calculator's memory. For example, if you want to store the value 10 in the calculator's memory, you can press the "M+" button after entering the value. Then, if you want to add 5 to this value, you can enter 5 and press the "M+" button again, and the calculator will add 5 to the value stored in memory, giving a result of 15. If you want to recall the value stored in memory, you can press the "MR" button. If you want to clear the memory, you can press the "MC" button.