electric field lines
Electric field lines are a drawing trick that makes an invisible electric field visible. They are imaginary lines you sketch so that, at every point, the line's direction shows which way the field points, and how crowded the lines are shows how strong the field is. Iron filings around a magnet give a similar picture for magnetism; field lines do the same job for electric fields on paper.
The rules are simple and precise. Field lines start on positive charges and end on negative charges (or run off to infinity). The arrow on a line points the way a positive test charge would be pushed. Where lines are packed close together the field is strong; where they spread out it is weak. Lines never cross, because the field has one definite direction at each point. The number of lines leaving a charge is drawn proportional to the amount of charge.
Field lines are a visualization, not real objects, but they capture the physics beautifully. A single positive charge has lines radiating straight outward like spokes; two opposite charges (a dipole) have lines curving gracefully from the positive to the negative. Because a field line always crosses a surface of constant potential at a right angle, the lines and the equipotential surfaces together form a complete map of the electric situation.
Around a single positive point charge, draw lines pointing straight out in all directions; they thin out with distance, mirroring how the field weakens as 1/r^2.
Denser lines mean a stronger field; the spreading of the spokes shows the field fading with distance.
Field lines never cross. If two lines met, the field would point two ways at once at that point, which is impossible.