Its the act of willing abstinence or reduction of food and water for the particular period of time. Usually this occurs due to the religious reasons. For example "Ramadan"

"I'm fasting - do you know what fasting means?" Has Ramadan started already? I thought it was still a couple of weeks away. There are both religious and non-religious reasons for fasting. It is a well-known concept, even if people don't often do it. Have you never thought about the word "breakfast"? :)

If the person who answered was a Muslim, it means that she abstains from food,drink and sex from dawn(before sunrise about an hour and a half roughly) to sunset. It is an act of worship to God. This act has been a must for 30 days in a month called Ramadan in the Muslim calender during the year. A person can volunteer to do so on any other day if he/she wants to further reward from God. Fasting isn't a must for everyone for e.g. children (below the age of puberty) aren't obliged to do so, they're totally free. Sick or travelling people as well are allowed to fast another day instead of the ones they needed to eat and drink in. Pregnant lie in the same category of the sick. There are other categories too, that are prohibited to fast, even if it's Ramadan for their health concern, and they too make up for the amount of days they hadn't fasted at any time of the year. The spiritual meaning behind this act which I get myself learned is patience on developing habits, it makes it crystal clear that procrastination is a mere excuse to run away from realizing the best of one's manners and success. There is something else special about this act that in Islamic religion the amount of reward isn't countable instantly, but because it varies in efficiency from one person to another based on his sincerity and honesty as regard to moral part, the reward is rewarded by God on the day of rewarding(judgement day). If the person was a Christian, he abstains from food of animal source and it's better to ask a christian about that. Regarding fasting in Islam this answer is a short, simplified one, there are details that I haven't mentioned not because they're uunimportant, butbecause this act of worship in Islam has is one of the five pillars in the religion . My answer is a personal simplified answer regarding fasting in Islam.

Fasting and prayer for short periods of time is done also according to the Bible. Thus Christians, Messianics, and Jews will also fast at different times for different reasons. People may fast also for health reasons, for a limited time, for example as a part of a body cleanse before starting a special diet, or before going to the hospital for a specific medical procedure. Fasting is abstinence from food and sometimes certain fluids also, you can fast by not eating yet you still drink water, or juice. It all depends on what type of fast you do.

Do you mind if I correct something? You should just say 'I'm fasting' - you don't need the word 'in'.