"The electric cone in the ground"

If a lighting bolt strikes the ground, there will be electricity which spreads into the ground. Surrounding the spot where the lightning bolt struck the ground, there will be a very strong electric field for the blink of an eye. Lightning bolts transport electrically charged particles between the ground and a cloud in the sky. An electric current flows. The result of this is an electric field in the ground around the spot where the lightning bolt struck. Along the surface of the ground this fields spreads circularly and downwards into the ground the field spreads conically. The strength of the lightning bolt determines the size of the circular area and the how deep into the ground the cone reaches. Apart from the strength of the lightning bolt, further determining factors for the field are the humidity and the composition of the soil.

The cone-shaped electric field describes the resulting voltage at each point in the surrounding area caused by the lightning bolt. Every point in the fields needs to be compared to one set reference point (e.g. the spot where the bolt struck) in order to determine its voltage. It all gets very clear if all points of same voltage are connected. The result will be a line. Along this line all points have the same electrical potential, i.e. voltage. Because of this such lines are called equipotential lines. You can draw such lines, if you take care that the voltage-difference between two neighboring lines is always the same. Equipotential lines always run vertically to the field lines.
If two lines with different voltage-values are connected with a conductor (even a weak one), an electric circuit is the result and an electric current will start to flow. This current can then, for example, cause a strong rise in temperature and thereby cause a fire.

If we talk about a "lightning strike", we often do not only mean the direct spot where the bolt struck the ground, but we also mean the effects which are caused by the electric field and its voltage.