Ash Wednesday is when the Church begins the forty day journey towards the Triduum (The Three Days), with the annual discipline of fasting, & giving Alms.
Ash Wednesday & Good Friday (April 10) are days of fast & abstinence. All Fridays of Lent (February 28, March 6, 13, 20, 27, & April 3) are days of abstinence. Fasting refers to only one full meal, with two small meals & not eating otherwise for the day. Abstinence refers to not eating meat at all during the day. The Church directs the Holy Fast to all those in their 18th year until their 60th year (Canon 1252).
The ancient practice of fasting & abstaining can occur throughout other days in Lent. This is not obligatory & can take a variety of forms distinct from food.