Mass is how much there is of an object. Mass is related to how much something weighs. Mass and weight are two different things. The unit for mass is a gram. A nickel has the mass of about one gram.
Objects that take up space and have mass are called matter. Everything around you is made up of matter. Chocolate cake is made up of matter. You are made of matter.
If you are having trouble understanding matter, look all around you. You can see matter makes up the walls of your house and your classroom. Matter is large and matter is small.