I don't understand this question. Is it asking the for the method's signature, which is:
public int compareTo(Object o)
or is it: compareTo() returns negative numbers, 0, and positive numbers respectively indicating whether the calling object is less than, equal to, or greater than the specified object?
The instructor is looking for that second answer -- what the return value means. That's the semantics ("meaning") of a method. The signature is the "syntax".