«Singular» (مفرد) refers to just one thing, like: a pen, a book, a flower. «Plural» (جمع) refers to more than two, like: pens, books, or flowers.
Arabic also has a «dual» form (مثنى) for exactly two things, like «two pens» (قلمان). So Arabic has three number forms: singular (one), dual (two), and plural (three or more).
Some plurals are made by adding letters at the end of a word, like «معلم» (teacher) becoming «معلمون» (teachers). Others change shape completely, like «كتاب» (book) becoming «كُتُب» (books). This kind is called a «broken plural» because the word's shape breaks and changes.
