You are on the surface of the Earth now (I assume), as are almost all other humans. The surface of the Earth can be thought of as a two dimensional plane.
OK, let's say you do something amazing and you want to tell everyone on Earth about it, so you set up a means of broadcasting your news, at the speed of light, across the surface of the Earth. The speed of light is quick, but it still takes nearly two tenths of a second for your signal to make it round the world.
Before that signal gets to them, nobody knows your news. You can imagine a circle, defined by the speed of light times the travel time, spreading outward from your location, containing those regions that have heard the news.
Now what you do is imagine those circles stacked on top of each other, each drawn after a small increment in time, with a larger radius, and separated vertically by an amount that represents that increment in time. The circles begin at your position, when you start to broadcast. This stack of circles of increasing radius forms a light cone. Inside the cone are regions of space and time that it is possible for you to communicate your news to. Outside the cone are regions (of space and time) that can never hear the news because no signal can get there faster than the speed of light.
A concrete example. It is about 5000 km between London and New York. If the stock market crashes at 17:00 (UT) in New York, it is impossible for traders in London to hear about it for at least 0.016 seconds. For that period, they lie outside the light cone of a signal produced at 17:00 in New York. More distant cities take even longer to intercept the light cone.
The light cone idea also works in reverse. You can invert the cone to mark regions of space and time from which you could have heard some news. In the example above, the inverted light cone of the London traders does not contain New York until 0.016 seconds after 17:00.
A point of confusion is the idea of a cone, which is really only appropriate if space is defined as two dimensional - e.g. points on the surface of the Earth. Conceptually it is harder to work with in 3D (though mathematically equivalent). In 1D (points along a line), the light cone approximates to a light triangle.