Social inequality can help to contribute to poverty because people in the lower strata of the society tend to have fewer opportunities to get ahead.
In the United States, for example, social inequality tends to include educational inequality. The children of people in the lower stratum of society end up attending schools that have fewer resources and typically provide a lesser standard of education. This, in turn, makes those children more likely to end up in poverty. Their education is substandard and so they cannot get into good colleges (or perhaps into any college) and therefore cannot get good jobs.
Because social inequality leads to unequal opportunities, it helps to cause and perpetuate poverty.