A cone is a shape with a round bottom that gets smaller as it goes until it reaches a point. If you cut a slice through a cone parallel to the bottom, it will be the same shape as the bottom. If you cut a slice through the cone vertically going through the top point, you will get a triangle.
Many things are cone shaped. volcanos tend to be cone-shaped, pinecones and pine trees like the ones in the picture are cone-shaped, and of course traffic cones are cone shaped.