When we want to say something is small in size, or show affection toward it, Arabic uses a special pattern called «diminutive» (تصغير). We take the normal word and reshape it slightly, usually into the pattern «فُعَيْل».
For example: «كتاب» (book) becomes «كُتَيِّب» (small book). «ولد» (boy) becomes «وُلَيْد» (little boy). «بيت» (house) becomes «بُيَيْت» (small house). Notice that every diminutive word starts with a damma, then an open letter, then a silent yaa.
We also use diminutives to express affection, like adding warmth when speaking to a small child, which makes the words feel gentler and kinder.