What is what does m- mean on a calculator?

M- on a calculator stands for "Memory Minus".

It's a function that subtracts the currently displayed number from the value stored in the calculator's memory. The link to calculator's memory is here. Think of it as an accumulator; it keeps a running total. If there's no value in memory initially, M- will still subtract the displayed number from zero, effectively storing the negative of that number in memory. After using M-, you can recall the value in Memory Recall (MR) .