As mvgulik pointed out, NoSQL is a quite broad term to classify all non-relational database management systems.
Myself, I'm not sure what 'knowledge of NoSQL' means in general. For me it is a knowledge of the particular database engine. Its use-cases, upsides and downsides, limitations, etc. Something like here: http://kkovacs.eu/cassandra-vs-mongodb- ... b-vs-redis. Maybe on the basic level it means the knowledge of various data structures that are commonly used in NoSQL databases.
Anyway, to get more grasp I'd recommend this book: http://martinfowler.com/books/nosql.html