The "field summary" of the generated Javadoc page is intended to summarize (usually open and protected) class fields and provide links to a more detailed description of the fields further down the page. As with methods, the text in the summary section for fields consists of the first sentence of the associated detailed section.
Since Java has developed good practice, this means that mutable fields are usually private, usually constants that are documented there (e.g. java.lang.Double has several), although there are some classes that have modifiable fields (e.g. javax.xml.ws.Holder has one, but the class is simple enough that this is not a problem).