Please explain the meaning of this passage in terms that are easy to understand.
Many of the regions larger political unit had declined and the absense of strong, stable, political unit made it more difficult to resist the slave trade.
This passage appears to be referring to West Africa during the time of the Atlantic slave trade. It is trying to explain to you why the countries of West Africa did not resist the slave trade.
The first part means that the "countries" of the area had become weak. The passage uses the term "larger political units" because these were not exactly countries in the way we think of them today. However, these were countries in the sense that they were independent of any higher authority.
The second part means that the weakness of these countries made it hard to resist the slave trade. If the countries had been stronger, they would have had a better chance at fighting back against the Europeans and Africans who came for slaves. But because they were weakened, they were not able to fight back.