If you speak a European language, the root system of Arabic may be an unfamiliar concept. Arabic words are made from a few component letters, commonly called a "root". A root usually consists of three letters (sometimes 2 or 4), which convey a basic idea. By adding various vowels (i.e. changing pronunciation) as affixes*, associated meanings can be derived.
Root words are the Arabic letters that form the main part of a word. These root letters provide the basic meaning of a word. The root letters are joined together with other letters to form different words which have related meanings.
Example: the root of a plant will determine the type of plant, or the genes of a person determine the characteristics of a person, so similarly the root word will determine the meaning of the word.