Fasting is the act of deliberately denying yourself something you are fond of for a desired purpose.
One can fast for health reasons, or do a distraction fast to improve focus.
To the believer, fasting is choosing to separate yourself for a season for a Godly cause. It is deciding to starve your flesh of things it is used to, so its desires are suppressed and your spirit can operate at maximal capacity.
It is a weapon of warfare. Daniel and Queen Esther employed it as part of their prayer and supplication unto God in their time of dire need. Whenever a fast was proclaimed among God's people, it was to restore focus and bring them into alignment with His divine purpose - Joel 2:15, Ezra 8:21.
It is done in obedience to God's call to draw nearer to Him. In the sober state fasting brings us to, we become more aware of His presence. Whether from food, water, or certain engagements, fasting is God's invitation to turn attention away from ourselves and unto Him.
It is a tool for self- discipline, not a bribe or blackmail to hold God to ransom. It works ON you and FOR you. It is not an exchange to get your desires from God.
In true fasting, your heart and body are invested to yield a desired result. Jesus separated Himself for a season in fasting and prayer to be alone with God in preparation for the assignment ahead.
God expects us to fast. Matthew 6:16-18 says "WHEN ye fast", not "IF ye fast". It is part of the believer's code for life. When we fast, we bring our hearts, minds and bodies to focus on seeking the Lord.
Fasting is not punishment. It is an invitation to draw near to God.