Simple Definition:
Catenation is the process of linking atoms of the same element together to form chains or rings [circular shapes]. Carbon [a type of element] has a unique property that allows it to form long chains or rings with itself. For example, carbon atoms can catenate to form straight chains like in the compound ethane [a type of gas used for heating and cooking], or they can form rings like in the compound benzene [a common organic solvent].
Very Simple Definition:
Catenation is when atoms of the same element stick together to make long chains or circles. Carbon is a special element that can make long chains or circles with other carbon atoms. For example, carbon atoms can stick together to make a long chain like in ethane [a type of gas used for heating and cooking], or they can stick together to make a circle like in benzene [a type of liquid that burns easily].