It Expanded What Counts as a 'Treaty'
Shabtai Rosenne, who is both a professor of international law and a former ambassador, published an analysis of this (Rosenne, 1995). Much of international law is based on formal documents such as signed treaties, agreements, etc. In the Qatar/Bahrain case, the court found that a series of letters exchanged by officials from the two countries constituted a valid agreement under international law.
To focus on the context of the case, both Qatar and Bahrain claimed sovereignty over some islands and sea-space. Although there was no treaty or protocol dictating how they should solve this problem, the King of Saudi Arabia had been acting as a mediator through a series of letters.
These letters were being sent by state officials in Qatar and Bahrain to the government of Saudi Arabia. In one of those letters, the two states agree that should a disagreement arise over this territory the International Court should resolve it.
Qatar would later claim that these letters are a valid international treaty. Bahrain claimed that they were not. The court agreed with Qatar and decided that they had the ability to intervene in this case.
The Wikipedia Quote
The Wikipedia quote in the question seems to be referring to the legal status of the letters. When they were signed they presumably represented some kind of shared understanding between Qatar, Bahrain, and Saudi Arabia. However, once Qatar disputed control over some of the territory covered in those letters the entire legal status of that agreement was contested.
Rosenne, Shabtai. 1995. "The Qatar/Bahrain Case: What is a Treaty? A Framework Agreement and the Seising of the Court". Leiden Journal of International Law, 8 (1): 161-182. Link.