Sunnah means way or practice. In Islamic terminology, it refers to any act performed, verbally encouraged, or sanctioned by the Messenger of Allah (PBUH) by being performed in his presence or knowledge and not being disapproved of. This is also known as Hadith.
The wisdom of the Prophet (PBUH) since the dawn of his mission, served to inspire, guide, enlighten, and heal millions through the journey of life.
The book in your hands is a compilation of a hundred Ahadith (plural of Hadith), of mostly the second type of Sunnah, words of advice and teachings related to man and his relationship with his Maker, fellow human beings, and himself, i.e. something which can be implicated in day to day life.
The main purpose of this compilation is that the reader is able to memorize and practice the Ahadith. The Messenger of Allah (PBUH) said,
"He who learns forty Ahadith with regards to my Ummah,
in relationship to religious injunctions,
Allah will raise him as a Faqih (Jurist),
and I will be an intercessor and a witness for him
on the Day of Judgment." al Bayhaqi in Shuabul Eman