Put simply it is to travel from their place of departure to their point of arrival in a well maintained and safe aircraft. They can expect to have flight deck crew trained to fly the aircraft and cabin crew trained in all the safety aspects of the cabin. They should expect any hold baggage to arrive with them at their destination.
If it were that simple, airlines could save massive amounts of money as there would be no need for them to advertise with the aim of differentiating their product from the competition's.