Prefixes are groups of letters added to the start of a word, e.g. "prefix". Prefixes usually qualify or change the meaning of the word. For instance, the word "television" literally means "far sight" or, more accurately, "sight from afar".
You will come across a whole host of prefixes in everyday life. "Understand", "octopus", "mishear", "reorganise", "photographer", "immobile" and countless other words are formed with prefixes. Knowing this and looking closely at how words are formed will help you to spell better because you can break the word down into its parts before spelling. It also helps you to understand what you are reading. You might never have seen a "quadrangle", for example, but if you know that words formed from "angle" usually refer to shapes and the prefix quad- means four, then you can easily picture a four-sided shape.
Make an effort to learn new and unfamiliar prefixes in order to broaden your vocabulary.
See how well you do in this quiz on the subject.