The short answer is that the Battle of Kosovo represents the heroic last stand of Serbia against the overwhelming Ottoman invasion. But to further explain this, some backstory is needed.

After the death of Serbian emperor Stefan Uroš IV Dušan (also known as Dušan the Mighty), his 20 yo son Stefan Uroš V (also known as 'the Weak') inherited the throne. At that time Serbia had some powerful nobles, and while his father managed to control them during his reign, Uroš V was not such a strong leader. And the nobles knew that. So, during his reign, there were a lot of internal conflicts between him and the nobles. Even his uncle Simeon Uroš rebelled against him and proclaimed himself Emperor of Greeks, Serbs and Albanians. So, in this prolonged period of internal struggles, the empire slowly started to weaken. The final collapse of the Serbian empire came when emperor Stefan Uroš V died. Since he had no heir, the nobles that rebelled against him took their chance and split the empire into many smaller states.

When the Ottomans started their expansion into Serbia, many of these newly formed kingdoms were too weak to oppose the Ottomans, so they willingly became vassals of the Ottomans. As the Ottomans advanced further, the prince of Moravian Serbia (the largest of the newly formed Serbian states) Lazar Hrebljanović, Vuk Branković (a Serbian noble who ruled over Kosovo) and Vlatko Vuković (commander of the Bosnian forces sent by King Tvrtko I Kotromanić of Bosnia) met the Ottoman army, led by Bayezid I, in battle on Kosovo Polje. In the battle both armies were obliterated, and both Lazar and Bayezid died. Even tho most sources say that the battle had no clear winner and that both armies just quit the field at one point, it is still considered an Ottoman victory because Serbia did not have enough fighting men after the battle to properly defend itself against another Ottoman attack.