The madd means to prolong (stretch) the sound of a vowel. There are too many rules of madd, you will learn The Letters of Madd and The Elongation Signs of Madd in this page.
There are three letters of Madd, which are ا – و – ى
These letters prolongate of a previous vowel to the measure of two vowels (2 counts), which is equal to the measure of one alif *.
It is not possible to read a madd letter alone and these letters (long vowels) only elongate the letter by the Alif with no Harakat.
* One Alif Amount: It is the time passed, to raise one finger (without dropping) or when we spell one alif letter or twice the time spent for one haraka (Example: as much as ha –> haa)