Air is a gas.  Air has oxygen.  Air is all around us.  The atmosphere around Earth has air.

An air mass is a big group of air molecules.  Molecules are tiny things.  Air is made of air molecules.

A mass is a group of something.

A molecule is a tiny. thing. We cannot see molecules with our eyes.  Different kinds of molecules make many kinds of things!

A molecule is so small we cannot see it.  There are different kinds of molecules.  Air molecules make air!

A front is when two air masses push against each other.  An air mass is a very big piece of air.  An air mass can be warm.  An air mass can be cold.  A warm air mass and a cold air mass can push against each other.  This makes a front.  A front can change the weather!