what are coplanar lines?

Lines (in three dimensions) that occupy the same plane are coplanar. That will be the case if they intersect somewhere.

Not all lines in three dimensions intersect.

Lines in three dimensions cannot be defined by a single equation. The general form is
(x-x1)/A = (y-y1)/B = (z-z1)/C
where A, B and C are constants, and (x1,y1,z1) are the coordinates of any point on the line.