Shadda is a diacritical mark, indicated with a small “w” shaped mark above Arabic letters. A shadda indicates a double consonant or long consonant.

شَقّة (shaqa) apartment - with a shadda on the qaf (ق)

شُقَق (shuqaq) apartments (pl) - without a shadda

The presence of a shadda can cause a word to take on a very different meaning, than what it would be without the shadda. For example, “darasa” (دَرَسَ) means he studied. “darrasa” (دَرَّسَ) means he taught.

- darasa

- darrasa

If there is a damma or fatha, along with the shadda, those go above the shadda. If there is a kasra on the same letter, the kasra goes beneath the shadda, with both above the letter.