When two planes intersect, they form a line. This line is the set of points that are common to both planes. To visualize this, imagine a flat sheet of paper (representing one plane) and a second sheet of paper that is tilted and passes through the first sheet. The edge where the two sheets meet is analogous to the line of intersection.
The intersection can also be understood geometrically: if two planes are parallel, they will never intersect. However, if they are not parallel, they will always meet along a straight line. This line is infinite in length, extending in both directions, as it continues wherever the planes intersect.