Fasting is when you abstain from eating food for a period of time longer than you normally would.

In the Bible, people would fast during times of mourning, affliction & persecution.

Typical fasts last 24 hours.

If you fast long enough you will feel a sense of well being & clear mindedness.
I notice these effects begin after around 17 hours of fasting.

Jesus said that his disciples would not fast while He was with them, but when
He was taken away, then they would fast (Matthew 9:15).

People often fast in times of mourning, repentance & gratitude. As well as to seek
God's council or to be able to offer more fervent and acceptable prayer.

In Matthew 17:21 Jesus tells us that some demons will not leave without fasting and prayer.
"Howbeit this kind goeth not out but by prayer and fasting."