Matter is the substance of which all material is made. That means objects which have mass. More specifically, they must have "rest" mass, which is a form of energy that matter has even when it is not moving (it has no kinetic energy), is extremely cold (it has no thermal energy), etc. "Matter" is a word that is sometimes used in varying ways in everyday life, whereas "mass" is a well-defined concept and quantity at least in physics. They are not the same thing, though they are related.