what is qalqalah?

Question
what is qalqalah?

Answer
The qalqalah is defined as a disturbance.  It is a characteristic found in the letters of the group: when they have no vowel (are saakin).  The letter is then pronounced not completely saakin nor is it pronounced with a vowel, instead it sort of "bounces."

You can read more about how the qalqalah is formed in the following link:
http://www.abouttajweed.com/the-qalqalah-as-sakt-and-al-ikhtilaas/the-qalqalah-mechanism.html