The Discovery component is about discovering, or finding, implementations for pluggable interfaces. It provides facilities for instantiating classes in general, and for lifecycle management of singleton (factory) classes.
Fundamentally, Discovery locates classes that implement a given Java interface. The discovery pattern, though not necessarily this package, is used in many projects including JAXP (SaxParserFactory and others) and commons-logging (LogFactory). By extracting this pattern, other projects can (re)use it and take advantage of improvements to the pattern as Discovery evolves.