Some places ask for a query letter before submitting.  A query letter is normally a succinct note from you telling the publisher what you've done.  If they are interested, then they'll tell you to go ahead and send the material.  Normally, a query letter is not needed when submitting poems and stories, especially in response to an ad.  A query letter is, however, a good idea if you've done an interview, review, or something out of the ordinary, and certainly if you are suggesting a major writing project such as a novel or a book of poems or short stories.