A departure tax is a fee charged (under various names) by a country when a person is leaving the country.
Some countries charge a departure tax only when a person is leaving by air. Various rules apply to the payment of the tax, including payment at the airport to those about to catch a flight, or by some prepayment method, or it may be charged to the airlines and included in the airline ticket price.
For specific inforamtion per country please see the following link:  https://www.gov.uk/foreign-travel-advice