A substance is homogeneous if its composition is identical wherever you sample it - it has uniform composition and properties throughout. Homogeneous is Latin for the same kind. If a substance is not homogeneous, it is said to be heterogeneous.
Chemical elements can be homogeneous. Examples of homogeneous elements are: nitrogen in a balloon, mercury in a thermometer, or gold in an ingot.
Compounds can be homogeneous. Examples of homogeneous compounds are: carbon dioxide in a balloon, water in a bottle, or plastic forming an electric socket.
Mixtures can be homogeneous. Examples of homogeneous mixtures are: air in a balloon, salt water in a bottle, or brass in an ingot.
Chemical elements need not be homogeneous. For example, a sample of tin suffering from tin pest is not a homogeneous element.
Compounds need not be homogeneous. A sample of water that is slushy (partially frozen) is not a homogeneous compound.
Mixtures need not be homogeneous. A teaspoonful of sand in water is not a homogeneous mixture.