It was in regards to the Maastricht Treaty in 1992, which was sort of the founding treaty of EU. In order to complete the transformation from European Communities to a European Union, all involved countries had to sign the treaty.
Some of the countries just couldn’t legally give off this amount of sovereignty without a domestic election. Denmark being one of them. So, even if it’s a small country, it had the same power as any other country to obstruct the founding of the entire union.
So when the election turned out a narrow “no”, it was a bit of wrench in the wheel. Denmark then negotiated having a few opt outs before they were able hold a new election which then gave a “yes”.