Almost every Arabic word – except proper nouns and foreign words – has a root that consists of three or four letters, so called radicals. In Arabic, a radical is called حَرْف أَصْلِيّ.
It is pretty easy to identify the root (جِذْر) as there are only ten letters in Arabic which can be added to a root. They are called حُرُوف الزِّيادة
You can remember them easily because they can be summed up in this word: سألتمونيها
Difficult to remember? Well, think of a possible pronunciation, and it will be easy: “sa’altumuuniiha”.