Descriptive language is vivid and specific, and helps someone imagine a scene he didn't witness. You probably hope that your friend who works at the morgue isn't too descriptive when you ask him how his day was.
The word descriptive comes from the Latin descript-, meaning "written down." Something that is descriptive uses an account of words to give us a sense of what it's like. We usually use descriptive when people are very effective at conveying something. James Agee is an author who excels at descriptive writing, spending pages and pages beautifully describing the scene of a poor cabin in the moonlight.