A perpendicular line is one that intersects another line at a right angle, which is 90 degrees. In geometric terms, when two lines are perpendicular, they meet to form four right angles at the point of intersection.
Visually, if you were to draw one straight line on a piece of paper, and then draw another line that crosses it at a perfect right angle (like the corner of a square or rectangle), those two lines are perpendicular. For example, if one line is horizontal and the other is vertical, they are perpendicular to each other.
You might also see perpendicular lines marked in diagrams with small square symbols at the intersection point, indicating that the angle formed is indeed a right angle.