"Macedonia" refers to a historic region that has vague boundaries that can include Albania, Bulgaria, Greece, North Macedonia, Kosovo, and Serbia. The word also refers to Macedon, Alexander the Great's ancient Greek kingdom. Long after Alexander the Great's death, Slavic people related to the Serbs, Croats, and other neighboring peoples moved to the area, and eventually formed their own "Macedonian" culture. When Yugoslavia was founded after World War I, what is now the country of North Macedonia was formed, as one of Yugoslavia's six parts. After Yugoslavia broke up, it became its own country, the Republic of Macedonia.
Greek nationalists consider themselves to be the descendants of Alexander the Great and Macedon, and see Macedonia as a threat to this legacy, as the country’s name could be seen as implying that the modern country of Macedonia is Alexander’s modern heir. Greeks also feared that Macedonia could try to claim the neighboring territory of “Greek Macedonia,” which sits within Greece. As absurd as this may seem, the Macedonian government did not help things by building a grand neoclassical downtown in Skopje, complete with a giant statue of Alexander the Great! Macedonia found a compromise by officially calling itself the awkward term "the former Yugoslav Republic of Macedonia" (FYROM). This term was used everywhere from the UN to the EU and NATO.
In 2018, Macedonia and Greece came to an agreement: Macedonia would change its name, and Greece would finally let the former start the process of joining NATO. Thanks to this treaty, as of February 2019, Macedonia is now the Republic of North Macedonia. North Macedonians still use “Macedonia” in conversation, and this remains a bad topic to bring up in Greece.