toString is not a keyword  in Java. It is a method of class Object,
therefore a method of all classes (since Object is the base class in
Java hierarchy). Usually it returns a textual representation of the
class.

Form JDK doc:

"Returns a string representation of the object. In general, the
toString method returns a string that "textually represents" this
object. The result should be a concise but informative representation
that is easy for a person to read. It is recommended that all
subclasses override this method.
The toString method for class Object returns a string consisting of
the name of the class of which the object is an instance, the at-sign
character `@', and the unsigned hexadecimal representation of the
hash code of the object. In other words, this method returns a string
equal to the value of:

getClass().getName() + '@' + Integer.toHexString(hashCode())"

BTW, it will be a really good idea to install the JDK documentation
and actually read it :)

Generaly it displays some sort of textual refference of what is
contained in that object. Sometimes however it only reffers to a
Objects HashID and memory information.