In English, adding an 's' to the end of a word makes it plural (three or more things). The easiest Arabic plurals work exactly the same way! They are called "Sound Plurals" (Saalim) because the original word stays perfectly intact, we simply add a suffix at the end!
Men get one suffix (`-oona`), women get another (`-aatun`). Notice how the base word (مُسْلِم) never breaks or changes its inner vowels.
Remember the Mirror Rule? If a noun is Plural, its adjective MUST also take the exactly matching plural suffix (`-oona` or `-aatun`)!