Hamzah al wasel (connective hamzah): when the letter ا comes between two words, we connect the two words together and the hamzah sound cannot be heard. For example, although we can see the ا in صباح الخير, we do not pronounce it. The ح in صباح connects directly to the ل in الخير.
Pay attention to the sun and moon letters. If it is a sun letter, the ا and ل disappear and if it is a moon letter, only the ا disappears. For example, in صباح الخير only the ا cannot be heard, whereas in صباح النور both the ا and ل cannot be heard.
Listen to the following audios and notice where you hear hamzat-al-wasel.