A BNL is a type of Domain Specific Language (DSL), which is focussed on being readable and maintainable by subject matter experts. While these experts might not necessarily write code with the BNL from scratch, they can understand and maintain the business logic implemented with it very easily. As with all DSLs, a BNL reduces the clutter of general purpose languages - which also makes it possible for code using a BNL to be both executable specification and documentation of business processes.
In the presentation, Jay shows the motivation behind BNLs - why he even thought there's a need for the term, instead of just using the term DSL. He proceeds to go through the design process of an actual BNL and how to implement it using Ruby, and reports his experiences on doing this for several real world projects.
Watch Jay Fields' presentation "Business Natural Languages Development in Ruby".