Molecules: These are groups of two or more atoms held together by chemical bonds. They're like the completed Lego structures you make out of individual blocks (atoms).
Mass: This is the amount of matter in an object. It's like weighing your completed Lego structure to see how much it weighs.
This is the amount of space that matter occupies. Imagine putting your Lego structure into a box - volume would be how much room it takes up inside that box.