Balsa wood boat. Small flat bottle. Green food colouring. Some materials are heavier than others, and some are larger than others. If you have a large light object, we say it is not very dense.
Density is a measure of how much weight there is in a certain volume.
A bowl of water, various objects. Take some objects and see if they sink or float. Try to guess what will happen before you put them in the water. Large, light objects, such as apples and balloons, float. Generally an object will float on water if it is less dense than water.