Simply put, when multicolor light (like white light) enters a new medium (like glass), the different colors bend at different angles. This spreading out of the colors is referred to as chromatic dispersion and means that the each color will reach focus at a slightly different point. Visibly, when you focus you will see the different colors slightly offset from each other, like a colorful ghosting.

If the telescope's design does not try to re-align the colors so that they reach focus at the same point, the telescope is called Achromatic. These are the simplest telescope designs and, not surprisingly, the earliest telescope designs like that used by Galileo were achromatic.