What is what does supplementary mean?

Supplementary means something that is added on to or completes something else. It is an additional or extra feature or item that is not essential but helps to enhance or complete something else. Supplementary materials or resources are used to support or complement the main materials or resources and to provide further information or explanation. In mathematics, supplementary angles are two angles whose sum is 180 degrees, while in education, supplementary learning materials are resources that are used to complement the primary course material.