Before understanding NSSA area you need to understand Stubby areas. To make it short, ABR are not allowed to inject type 5 LSA (external route) into stubby area.
NSSA is a Stubby area where one can inject external route into OSPF. However the LSA type generated will be type 7 instead of usual type 5 for external route because Stubby area do not allow type 5... At ABR the type 7 LSA is transformed to type 5 when injected into the backbone area 0.
There are also Totally Stubby and Totally NSSA areas. The only difference here is that the ABR cannot inject type 5 (external routes) and type 3 (summary) LSAs into the area.