Most English words are based on foreign words, and Latin and Greek in particular. An English word can be made up of three different parts: the root, a prefix and a suffix.
The root word is like the root of a plant -- it's the part of the word to which everything else is attached. Becoming familiar with common root words and their meanings will help you understand a number of other words based on those roots, even if you've never seen the word before.
The root "cred" comes from the Latin word "credere," which means "to believe." So words with this root usually have something to do with belief. For example, "incredulous" means "expressing disbelief," and "credible" means "believable."
Prefixes, such as "dis-" or "anti-," are added to the front of a root to form a new word. The new word will have a different meaning, but one that is related to the original root word.
Suffixes, such as "-ed" or "-able," are added to the end of a root word to form a new word. As with prefixes, adding a suffix changes the meaning from the original word.
Familiarity with common roots, prefixes and suffixes will make spelling, reading, writing and vocabulary much easier. Websites such as those listed below, as well as numerous root-based vocabulary books, provide exercises to help you improve your understanding.