Peace be upon you.
What is the Sunnah, and what does it mean?

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His Slave and Messenger.

In the Arabic language, the word Sunnah means "way." In fact, the Prophet, sallallaahu ‘alayhi wa sallam , said in a Hadeeth, on the authority of Abu Sa'eed Al-Khudri that Muslims will follow the way of Jews and Christians. However in terms of the Sharee‘ah, it refers to all that is narrated from the Prophet including his actions, sayings and whatever he  approved of, and thus it includes obligations and recommendations.

The Sunnah is the second source of Islamic legislation.  However, this does not mean it has a lower rank than the Quran, which is the first source. This is because the Sunnah is on the same level as the Quran in terms of deriving a ruling if it has an authentic chain of narrators that proves beyond doubt that the Hadeeth is attributed to the Prophet, sallallaahu ‘alayhi wa sallam. Moreover, the person who refers to the Quran needs only to verify that the verse he takes as evidence is actually relevant to the ruling, because it is absolutely authentic and Mutawaatir (continuously and consecutively related), but the person who refers to the Sunnah needs two things:

Firstly, the person needs to verify its authenticity, meaning the thing was actually said, done, etc… by the Prophet .

Secondly, its validity as evidence on the relevant ruling. This, of course, is the factor that makes referring to the Sunnah more difficult than the Quran.

Once it is proved that an act or saying of the Sunnah is authentic, then this is considered exactly like the Quran, meaning a person must believe it and follow its rulings. Allaah the Almighty Says (what means): {Allaah has sent down to you the Book (The Quran), and the Wisdom (i.e. the Prophet's Sunnah)} [Quran   4:113]

Also, it was narrated that the Prophet warned against those persons who call only for depending on the Quran and neglecting the Sunnah, explaining that both have the same rank. [Imaam Ahmad, Ibn Maajah and At-Tirmithi]

Allaah Knows best.