Why Is ‘Aqiqah Important?
Question

I heard so many times that when a Muslim boy is born, that two sheep should be slaughtered, and when a Muslim girl is born only one. 
First of all, I would like to know if it is true, I mean Sunnah or obligatory. If yes, I would really like to know why. I mean if there is a special reason. How is it explained by our beloved Prophet Muhammad (peace be upon him) then? Some sisters and me heard about this, but actually I really wasn’t sure if it is correct.
Praise be to Allah.Is ‘aqiqah compulsory?
This sacrifice which is done for the newborn is called ‘aqiqah . The scholars differed concerning the ruling on ‘aqiqah; some of them said that it is obligatory and others said that it is Sunnah muakkadah (a confirmed Sunnah). 
The scholars of the Standing Committee said: 
“‘Aqiqah is Sunnah muakkadah. For a boy two sheep should be sacrificed, each of which should meet the same conditions as a sheep for udhiya (sacrifice), and for a girl one sheep should be sacrificed. The sheep should be slaughtered on the seventh day, but if there is a delay it may be slaughtered at any time, and there is no sin in delaying it, although it is preferable to do it as soon as possible. And Allah is the Source of strength; may Allah send blessings and peace upon our Prophet Muhammad and upon his family and Companions.” 
There is an odd and weak (da’if) opinion which says that ‘aqiqah is makruh. 
Ibn al-Qayyim said: 
“Imam Ahmad said, according to a report narrated by Hanbal: “It was narrated that some people said that ‘aqiqah is makruh because it is something that belongs to the Jahiliyyah. He said, this is because of their lack of knowledge and the fact that they are not aware of the reports. 
The Prophet (peace and blessings of Allah be upon him) did ‘aqiqah for al-Hasan and al-Husayn, and his Companions also did it. These people say that it is something that belongs to the Jahiliyyah, but it is the Sunnah of the Messenger of Allah (peace and blessings of Allah be upon him), who said: ‘Every child is in pledge for his ‘aqiqah’. 
This hadith has a good isnad and was narrated by Abu Hurayrah from the Prophet (peace and blessings of Allah be upon him).” 
According to a report narrated by al-Athram, he said: “Concerning the ‘aqiqah , there are ahadith with isnads from the Prophet (peace and blessings of Allah be upon him) and from his Companions and the Tabi’in, but these people say that it is an act of Jahiliyyah.” And he smiled like one who is surprised.” (Tuhfat al-Mawdud, p. 45-46)
Why is ‘aqiqah important?
With regard to the purpose of the ‘aqiqah , it was mentioned in a hadith of the Prophet (peace and blessings of Allah be upon him):  “Every child is in pledge for his ‘aqiqah.” 
The scholars differed as to the meaning of this hadith. It was said that the meaning is that if the ‘aqiqah is not done for him and he dies in childhood, he will be prevented from interceding for his parents; or that the ‘aqiqah is a means of ridding the child of the Shaytan and protecting him from him. 
The child may miss out on something good if his parents neglect to do the ‘aqiqah even though that is not his action, just as when the parents have intercourse, if the father says ‘Bismillah” the Shaytan will not harm his child, and if he fails to do so the child will not have this protection. (See Zad al-Ma’ad, 2/325; al-Sharh al-Mumti’, 7/535) 
How to do ‘aqiqah
With regard to how it is to be done, the Prophet (peace and blessings of Allah be upon him) explained that, as may be seen in the following ahadith: 

Salman ibn ‘Amir al-Dabbi said: I heard the Messenger of Allah (peace and blessings of Allah be upon him) say: “ ‘Aqiqah is to be offered for a (newly born) boy, so slaughter (an animal) for him, and remove all dirt from him.” (Narrated by al-Bukhaari, 5154) 
‘Aishah (may Allah be pleased with her) narrated that the Messenger of Allah (peace and blessings of Allah be upon him) commanded them to sacrifice two similar sheep for a boy and one for a girl. (Narrated by al-Tirmidhi; he said, it is hasan sahih; Ibn Majah, 3163; Sahih al-Tirmidhi, 1221)
Umm Karaz narrated that she asked the Messenger of Allah (peace and blessings of Allah be upon him) about ‘aqiqah and he said: “For a boy two sheep and for a girl one, and it does not matter whether they are male or female.” (Narrated by al-Tirmidhi, 1516; he said, it is hasan sahih; al-Nasai, 4217; Abu Dawud, 2835; Ibn Majah, 3162, without the last part; Sahih Abi Dawud, 246) 
Samurah ibn Jundub narrated that the Prophet (peace and blessings of Allah be upon him) said: “Every child is in pledge for his ‘aqiqah, which should be sacrificed for him on the seventh day and his head should be shaved and he should be given a name.” (Narrated by al-Tirmidhi, 1522, he said it is hasan sahih; Ibn Majah, 3165; Sahih Ibn Majah, 2563) 

For more about ‘aqiqah, see this category: ‘Aqiqah and rulings on the new-born
And Allah knows best.