Qalqalah

The word Qalqalah in Arabic means vibration or shaking sound.
There are five Qalqalah letters: Qaaf ( ق ), Taa ( ط ), Baa ( ب ), Jeem ( ج), and Daal ( د ). If one of these five letters has a sukoon in the middle or at the end of a word, it makes a small vibrating or echoing sound when pronounced.

A simple way to remember them is by the phrase

قُطْبُ جَدٍ