In December 2007 the heads of state and government of the EU member states signed the Lisbon Treaty, which was designed to reform the way the Union functioned following two waves of enlargement that had almost doubled its size.
Article 50 is the part of that Treaty which sets out the procedure for any of the 28 member states to withdraw from the Union should they choose to do so.
It stipulates that any member state deciding to withdraw shall notify the European Council of its intention. The EU will then negotiate and conclude an agreement with that member state, setting out the arrangements for its withdrawal. The Treaties of the EU will cease to apply to the state in question from the date the withdrawal agreement comes into force or, failing that, two years after the notification of intent to leave. This period can be extended with the unanimous agreement of the European Council and the withdrawing member state.
This notification is what people mean when they talking about “triggering Article 50”.
This is also untested because no member state has done it before.