This week's book giveaway is in the OCAJP 8 forum. We're giving away four copies of OCA Java SE 8 Programmer I Study Guide and have Edward Finegan & Robert Liguori on-line! See this thread for details.
While a file with that same name might well be used in other situations, I'm familiar with its use as part of a Struts-based web application. In that situation, this file is used to store the String messages that are to be used on different web pages in different situations (for things such as labels and error messages).