Properties (and property files) are a feature of
Java in general; they have nothing in particular to do with servlets or JSP. They are often used for keeping system settings and preferences in files that can be maintained outside of the code. Some 3rd party libraries (e.g. log4j and Hibernate) make extensive use of them for their own configuration. It's more and more common to use XML for these purposes, though, which can accommodate structured information. But for simple name/value pairs, properties work well.
An introduction can be found
here.