The term "sun letter" refers to the Arabic letters that change their pronunciation when they are followed by certain letters. The opposite of sun letters are moon letters, which do not change their pronunciation. Some examples of moon letters include ب (ba), ج (jim), د (dal), ذ (dhal), ر (ra), ز (zay), س (sin), ش (shin), ص (sad), ض (dad), ط (ta), ظ (za), ٠(fa), ق (qaf), ك (kaf), ل (lam), م (mim), ن (nun), ه (ha), and و (waw). It is important to be aware of both sun and moon letters when studying Arabic pronunciation.