The root word is the part of the word that is key to its definition - after you remove any prefixes or suffixes. "Therm" means temperature. It is the root word for "thermal", "thermometer", "thermo-nuclear" - all words having something to do with temperature or heat. "Tele" means far. It is the root for "telephone", "teleportation", "telemarketing" and "telekinesis". All of these words mean to do something at a distance.

"Root" has a number of meanings in English (also some colloquial usages), but the meaning you want is what Fdmaxey described: the base unit where the meaning comes from in a word.

root = 根