What is a day of abstinence? A day of abstinence is a day on which we are not allowed the use of meat.
Who are obliged to observe the abstinence days of the Church? All Catholics who have passed their fourteenth birthday and have attained the use of reason are obliged to observe the abstinence days of the Church, unless excused or dispensed.
Why does the Church command us to fast and to abstain? The Church commands us to fast and to abstain in order that we may control the desires of the flesh, raise our minds more freely to God, and make satisfaction for sin.
But thou, when thou dost fast, anoint thy head wash thy face, so that thou mayest not be seen fasting by men, but by thy Father, who is in secret; and thy Father, who sees in secret, will reward thee. (Matthew 6:18)
More on that -> CCC 2189-2195