While upper type bounds limit a type to a subtype of another type, lower type bounds declare a type to be a supertype of another type. The term B >: A expresses that the type parameter B or the abstract type B refer to a supertype of type A. In most cases, A will be the type parameter of the class and B will be the type parameter of a method.
This is a companion discussion topic for the original entry at http://docs.scala-lang.org//tutorials/tour/lower-type-bounds.html.html