Advertising is a way of boosting the amount of jobs at an airport. Once you have payed for advertising, the job increase last for 8 hours. The larger the population of the city, the more expensive it is to advertise. The cost to advertise an airport is always however much the original airport cost, plus 10,000 more coins to advertise. To advertise, go to the airport info screen. Advertising is helpful when you're trying to get the full plane bonus and compete in Global Events.