Fasting is giving up something difficult, usually food if you are able. This can come in many forms, but whatever method of self denial you choose, this is a practice that leads us to depending on God as our ultimate source of life. Saying yes to Jesus means saying no to other things.

Self denial is telling yourself no so your spirit can grow.

“And when you fast, don’t make it obvious, as the hypocrites do, for they try to look miserable and disheveled so people will admire them for their fasting. I tell you the truth, that is the only reward they will ever get. But when you fast, comb your hair[a] and wash your face.  Then no one will notice that you are fasting, except your Father, who knows what you do in private. And your Father, who sees everything, will reward you." - Matthew 6:16-18