There are many patterns visible in the Arabic language.
Arabic words are composed of root consonants. Most words usually have 3 root consonants. For example the word كتاب (kitaab) shares the same ordering of root consonants with مكتب (maktab). Words that share the same ordering of root consonants are usually obviously related in meaning. The root consonants for the two mentioned words were k-t-b (note: order matters).
Templates are diagrams used to show where root consonants are in a word, it is a tool used to understand Arabic. The template for كِتاب (kitaab) is فِعال (CiCaaC). Note that كِتاب means book, while فِعال (CiCaaC) does not mean anything and is simply a tool used to understand Arabic morphology.
Notice that مَكْتَب (maktab) does not start with a root consonant, we know this because we know that it does not start with ك (k). The template for it is مَفْعَل (maCCaC).
Templates are an important tool for the learner of Arabic because it allows them to use a good Arabic dictionary. Using knowledge of templates can allow you to figure out the meaning of an unfamiliar word, without a dictionary. By understanding the relationship between words a dictionary is not needed. Much like a person learning English does not need to look up the word "cleaner" once they know the word "clean".
Arabic words are composed primarily of root consonants and a template. The root consonants contain the abstract meaning of the word, while the templates specify the meaning. For instance:( ك ـ ت ـ ب) k-t-b gives the abstract meaning of writing, because كَتَبَ ((kataba)) means "he wrote". With the template مَفْعَل (maCCaC), the consonants occupy the places reserved for them and make a word meaning "office": مَكْتَب (maktab).
Arabic uses this principle for verbs, nouns, and forming plurals. Notice that there are many different templates. The most useful ones are the noun templates. The template of place مَفْعَل (maCCaC), which we used to make a word meaning "office", can be seen used on signs of places in the Arab world. For exampe: مَطْعَم (maT`am) which means "restaurant".
Another advantage of templates is that words do not necessarily derive directly from other words. This prevents forming a noun from a verb unconventionally which happens in English (Best example: use of "presentate" instead of "present").