Simple Answer: Tariffs
A tariff is a tax (in this case) on imports. The South, particularly South Carolina, objected strongly to the high rate of taxation on goods she desparately needed. The rates did seem a little high -- 62% on 92% of the goods coming into South Carolina (and other southern states). For example if South Carolina want to import 1000 dollars worth of shovels, she would have to pay an additional 620 dollars to do it.
The acts of 1828 and 1832 were thought by the South to be ruinous because not only would she be forced to pay much more for basic needs, she would not be able to sell her cotton to Great Britain because of the way the tax was imposed.
Thus a very strong case was built for disobedience to the 1828 and 1832 acts. What made those two acts a pain was that North was determined to force unity on a South that had other economic problems during the 1830s (like drought). So the nullification process meant that something had to be done or South Carolina was threatening to go to war to protect her economy.
Thus the Tariff act of 1833 was introduced, and though you have not asked anything about that, I think you should note that Act was intended to unruffle South Carolina's feathers. It was a grand compromise devised by Andrew Jackson's administration. It succeeded until 1842 when it's tenure was up. You can read all of this by reading more about the Nullification Crisis. Be sure and read about Jackson's comments on it.
So this rather complex turn of events all really hinged on money and standard's of living. The vocabulary used was States Rights Vs Federal Rights. Put in very simple language: who has more rights, the one or the many? It took a civil war some 30 years later to resolve that question.
