Ash Wednesday marks the beginning of Lent for Catholics. The ashes we receive on our forehead in the shape of a cross serve as an outward sign of our sinfulness and need for penance. The ashes also symbolize our mortality, a reminder that one day we will die and our bodies will return to dust. Hence the traditional words, “Remember that thou art dust and unto dust thou shalt return.” 
The tradition of receiving ashes has its origins in the Old Testament, where sinners performed acts of public penance. It was Pope Urban II who in the 11th century recommended that all Catholics take part in the practice of receiving ashes on Ash Wednesday. In the 12th century it became customary that the ashes used on Ash Wednesday were made by burning the previous year’s palm branches.
Ash Wednesday is also a day of fasting and abstinence for Catholics. According to Church law, Catholics older than the age of 14 are to abstain from meat. In addition, those between the ages of 18 and 59, not including pregnant or nursing mothers, should eat only one full meal. Smaller amounts of food, not as much as a full meal, may be eaten in the morning and either at lunchtime or dinner, depending on when you eat your full meal.
(source: http://www.americancatholic.org/Messenger/Mar2003/Family.asp)
MORE INFORMATION ABOUT ABSTAINING FROM MEAT ALL YEAR
Canon 1250 states that the days and times of penance for the whole Church are the Fridays of the entire year, and the season of Lent. And canon 1251 gives further details on just how Catholics are to make these days penitential: Unless a solemnity falls on a Friday, abstinence from meat, or some other food as determined by the Bishops’ Conference, is to be observed on all Fridays; while Ash Wednesday and Good Friday are days of both abstinence and fasting.
(source: http://catholicexchange.com/are-catholics-supposed-to-abstain-from-meat-every-friday)
On the Fridays outside of Lent the U.S. bishops conference obtained the permission of the Holy See for Catholics in the US to substitute a penitential, or even a charitable, practice of their own choosing. They must do some penitential/charitable practice on these Fridays. For most people the easiest practice to consistently fulfill will be the traditional one, to abstain from meat on all Fridays of the year. During Lent abstinence from meat on Fridays is obligatory in the United States as elsewhere.
(source: https://www.ewtn.com/expert/answers/fast_and_abstinence.htm)
