coolant defines ANY liquid that flows through your cooling system and dissipates heat. Water alone can be your coolant. Or antifreeze mixed with water. Or Coca-Cola, though that formulation is probably not optimal.
Antifreeze is a solution meant to be mixed with water to lower its freezing point (and raise its boiling point). Mixed with water, you have a coolant that remains liquid in a wider temperature range than water alone. The stuff commonly used is ethylene glycol, an alternate is propylene glycol and there are other less common forms. You can't use the stuff by itself, it doesn't cool well. The bottle will indicate whether it has already been mixed, otherwise you need to mix it with the same volume of water. It's also not a good idea using water alone as antifreeze also has corossion inhibitors.
Antifreeze is a solution meant to be mixed with water to lower its freezing point (and raise its boiling point). Mixed with water, you have a coolant that remains liquid in a wider temperature range than water alone. The stuff commonly used is ethylene glycol, an alternate is propylene glycol and there are other less common forms. You can't use the stuff by itself, it doesn't cool well. The bottle will indicate whether it has already been mixed, otherwise you need to mix it with the same volume of water. It's also not a good idea using water alone as antifreeze also has corossion inhibitors.