Hi there! Thank you for the opportunity to help you with your question!
Coplanar are things that live on the same plane. Usually we use that when we are talking about points, lines and other geometric object; we say that three points are coplanar if they are all in the same plane.
One way to think about it intuitively, is to think of the plane as a sheet of metal (that you can't bend). You can imagine drawing three points on the sheet, in this case they would be coplanar. Alternatively, you can think of having two points on the sheet, and then a third point that is somewhere else in space, in which case they would not be coplanar.
The picture below, from mathresource, gives a visual idea of the concept:
Please let me know if you need any clarification. Always glad to help!