The package-info.java is a Java file that can be added to any Java source package. It is used to provide info at a "package" level as per its name.
It contains documentation and annotations used in the package.
javadoc example is already provided in the answer, the below part explains how it works incase of annotations.
For example, in the below file it is used to "substitute" the occurance of joda.time.DateTime with org.jadira.usertype.dateandtime.joda.PersistentDateTime
@TypeDefs({
@TypeDef(name = "PersistentDateTime", typeClass = PersistentDateTime.class, defaultForType=DateTime.class)})
package xyz.abc;
import org.hibernate.annotations.TypeDef;
import org.hibernate.annotations.TypeDefs;
import org.jadira.usertype.dateandtime.joda.PersistentDateTime;
import org.joda.time.DateTime;
There are a number of annotations available with which can be used to perform different things at "package" level. It can be found at https://docs.jboss.org/hibernate/orm/3.5/api/org/hibernate/annotations/package-summary.html