Perhaps we could describe in a little more detail.  A lot of the work App Developers do will involve operating on `Array`s and `Object`s, Underscore is a utility library that simplifies common operations you might perform on either an Array or Object.  A good example might be `_.map`, `_.filter` or `_.find`.  It might also be worth mentioning that it is possible to use ES5 Array and Object methods directly for a potential performance boost.