This is because a single lens works a bit like a prism. I'm sure you remember, from school, seeing white light being split into its separate colours by a prism. The single lens brings the different colours of light to a focus at slightly different points. Blue light closest to the lens, red light further away. It's acting like a lens and a prism. This is most undesirable for a telescope. It causes, what is technically known as, "chromatic aberration" This means that if we look at say the Moon, we would see different coloured fringes around the edges of the Moon.

To overcome this problem we can either make the telescope very long - a very very long focal length lens suffers only a tiny amount of chromatic aberration. But this would be quite impractical.

So modern refracting telescopes use a compound lens at the front of the tube, which consists of at least two lenses or elements, sometimes more. These elements when put together are known as the telescope's "objective lens". The simplest uses just two elements. The first lens or element is of the convex type (same as Grannys magnifying glass). This is followed very closely by a concave lens. Often the two lenses are bonded together, with a small air gap, so that they appear to be one lens. The idea is that both elements cause the undesirable effect, chromatic aberration, but in opposition to each other. And opposites cancel out (unlike the song!). This type of objective lens with just two elements is called an "achromatic lens".