There were two main issues addressed by the Northwest Ordinances. One of these was the issue of how new states would be allowed to join the Union. The other had to do with slavery in the Old Northwest.
The lesser of these two issues, at the time, was the issue of slavery. The Northwest Ordinances barred slavery from the areas that it covered. These were the areas that cover what is today Ohio, Indiana, Illinois, Michigan, Wisconsin, and part of Minnesota.
At the time, the bigger issue had to do with how new territories would be treated and how they would become states. This was a big deal because the people who wrote the law did not want a situation where the areas of the northwest became colonies of the established states. This might lead to a situation like that which caused the American Revolution. Therefore, they set up a system by which these areas could become official territories and, eventually, states.