A root, or root word is a wordwhich is used to form another word. It is also called a base word. Aroot does not have a prefix (A letter or group of letters added to thebeginning of a word) or a suffix (A letter or group of letters added tothe end of a word).
Root is the primary lexical unit of a word, and of a word family

For example, pure is a root. By adding prefixes and suffixes, the following words could be made:
impure, purity, pureness

Similarly, play and move are root words. By adding prefixes and suffixes, the following words could be made:
plays, played, playing, player
moving, movement, moved, movable, mover, movingly

In this book, I have given most common Greek and Latin roots which are used in English language