Complex sentence: The shape of an ellipse (how'elongated' it is) is represented by its eccentricity which for an ellipse can be any number from 0 (a circle) to just below 1.
Split into simpler sentences: The shape of an ellipse (how'stretched out' it is) is represented by its eccentricity. The eccentricity of an ellipse can be any number from 0 (a circle) to just below 1.