Structure

The second root letter is doubled and a sukoon is put on the extra letter (hence the shaddah).

فَعَلَ 
  ⇩
 فَعْعَلَ 
  ⇩
 فَعَّلَ   

Meaning & Function of Form II  Verbs:

*Form II verbs are nearly always transitive (المتعدي)

1) Form II to can be causative – it carries out an action on someone or something

E.g.    دَرَسَ  to study →  دَرَّسَ  to teach (i.e. make s.o study)

2) Form II can indicate the intensity or repetition of an action

e.g.
كَسَرَ he broke → كَسَّرَ he smashed
قَتَلَ he killed → قَتَّلَ he massacred
قَطَعَ he cut → قَطَّعَ he cut up [into pieces]

3) Judgement / Evaluation

صَدَقَ   he spoke the truth →  صَدَّقَ  he believed (considered s.o. truthful)
كَذَبَ  he lied →  كَذَّبَ he disbelieved (considered s.o. truthful)

4) Making Verbs from Nouns (nouns which don’t follow the Verbal Noun pattern تَفْعِيلٌ ) 

سُوْقٌ market → تَسْوِيقٌ he shopped
صُوْرَةٌ photograph → صَوَّرَ he photographed
خِيْمَةٌ a tent → خَيَّمَ he pitched a tent