Density is a measure of how tightly packed a substance is. If the particles are tightly packed within a material, it is denser than a substance where the particles are loosely packed. If both materials are the same volume, the denser one will weigh more. Have a look at the diagrams below, showing two substances of the same size:
Substances may be very dense because their molecules or atoms are heavy, or closely spaced, and because the substance is solid all the way through. Alternately, a substance may not be very dense, because the molecules are small, or farther apart, or because air or another light material is mixed in with the substance.
Aluminum, for example, is made from relatively small atoms, and they are arranged in a well-spaced pattern. Aluminum has a very low density for a metal ... given equal amounts of steel and aluminum, the aluminum will be much lighter.
Styrofoam is a solid foam-type material, which means it has lots of air pockets mixed with the plastic. Particles of air are very far apart, so the air doesn't weigh much. This makes the overall density of styrofoam very low.
Gold and lead are made from atoms which are very large, and they are tightly packed inside each piece of each metal. There are no air spaces inside the metals. As a result, these substances are very dense ... a small amount of each weighs much more than other substances.
Metamorphic rocks are dense; they are created by heat and pressure acting on less dense igneous or sedimentary rocks. Granite, for example, when compacted into a smaller volume, becomes gneiss. The metamorphic rock gneiss is more dense. (This also makes it more difficult to erode). Other examples of metamorphic rocks that are denser than the material that was compressed include schist (from basalt), quartzite (from sandstone), and marble (compressed limestone). Another example is diamond, which is denser than the coal which was compressed to create it.