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.
Volume: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.