These files are used by Spring to internationalize all strings in your module. It is highly recommended you use these.
The files should be in /omod/src/main/resources.
You need to list your messages.properties file in your config.xml file in "messages" elements.
The main english file should be named messages.properties. Each other language will be messages_fr.properties (for french)
Now in your jsp files you can use <spring:message code="mymodule.doSomething"/> and it will be translated when the user chooses to use the French locale.
What if I don't know another language?
Most developers only know a single language. It is acceptable to only fill out the english messages.properties file and wait for another language speaker to help you out.