A Charter flight is a flight that only takes place once or twice per week and is scheduled by a group of people who fly on that same airplane at the same time to the same destination.
The people who're flying on the charter flight book the flight together and pay for the tickets which saves money when you're chartering a flight.
A regular flight is a flight that takes off everyday of the week and where you purchase just one plane ticket per person who's flying with you and not a group of people.