To understand the events of 15 March 1848, we need to take a look at the years leading up to it. While Hungary has been an autonomous kingdom since the end of the 10th century, the crown belonged to the Habsburg dynasty since the middle of the 16th century. As a result, while Hungary did enjoy some autonomy, the actual ruler was the Habsburg emperor.
Over the course of the 19th century, more and more voices demanded fundamental reforms to make Hungary a modern country, with new options for development of the society and the economy. These would include the abolition of entailment and serfdom, as well as more options for receiving loans. Achievements of the era include the establishment of the Hungarian Academy of Sciences, the building of the Chain Bridge, and making Hungarian an official language of Hungary (it had been Latin up till 1844). The era is regularly referred to as the Reform Era.
However, reforms were not coming fast, while the ruling class was afraid that the reforms could destabilize their power (which was, of course, inevitable during a democratic, capitalistic transformation). In March 1848, revolts in Paris and Vienna encouraged ringleaders in Hungary to start a revolution of their own, and make the Parliament and the Emperor accept the Hungarian demands.