There needs to be some historical background on Hungary before I go on about the Battle of Mohács. Hungary was part of the Habsburg empire (Austria), though it had an impressive amount of freedom. As the Ottomans pushed to expand their empire, they came to the borders of Hungary. Dobzse László (Vladislas II) had dissolved the largest mercenary army in Europe and essentially dissolved his own power, as well as the country's unity before his death in 1516. György Dóbzsa led a peasant rebellion in 1514 that was brutally crushed by the nobility. The caused an internal division in Hungary, which did not help its precarious position between Vienna and the Ottoman Turks. His son, Lajos (Louis II) of Hungary ascended to the Bohemian and Hungarian thrones at the tender age of 10 (!) while under the guardianship of Maximillian I and later Georg von Hohenzollern.
The Turks had laid claim to Belgrade and Szabács, which meant Hungary had no defense to its south. King Lajos entered into a Habsburg alliance via marriage (Let others wage wars, but you, happy Austria, marry! as the motto goes) and this made the Ottomans nervous. Süleyman the Magnificent came to power and tried to strike a peace deal twice. Lajos didn't take either one and it's possible that he refused to do so because Ottomans raided and conquered even in peace time and figured a battle would offer a way out. Thus the Battle of Mohács happened.
I won't go into the details of battle (boring for me), but suffice it to say that Hungary lost, and lost Lajos as well (which Süleyman is said to have regretted; he did not intend to kill the young monarch). The Ottomans gained control of most of Hungary. The Habsburgs held onto western Hungary and Bohemia and the Kingdom of Hungary was broken up. The Turks tried to push towards Vienna, but were defeated every time. The interesting circularity of history dictated that the Ottomans should be defeated in the 1687 Battle of Mohács. Hungary laments the 1526 Battle of Mohács because it broke up Hungary and marked the decline of their kingdom and made their country a constant battlefield for 200 years. However, it must be said that Hungary did gain some power under the Austrian crown (they were co-kingdoms by the time it was dissolved after WWI). Despite this achievement, Hungary did not achieve independence until the 20th century. So perhaps the saying Több is veszett Mohácsnál (more was lost at Mohács) is accurate.