If you have booked or are looking to book a holiday abroad and going on an aeroplane then you may need to submit certain details from your passport at least 72 hours before the flight time. This is a separate requirement to getting a travel authority or a visa. In most cases you can provide your details online and you should be able to find out more information by contact the company you are flying with.
To help with security and safety the United States, most EU member states, and other countries now require airlines to provide details about their passengers before they travel. This is known as Advance Passenger Information. This often includes providing your passport number to your flight provider in advance of flying.
Although this may seem like another hassle and something else to remember it is important and if you do not do it you may find that you are unable to fly at that time. The idea of the system is to improve overall security to allow the airports information about who is flying prior to the departure and to have a record of who should be on which flight. It may also help them with security checks they may wish to do before you arrive at the airport.