What Leadfingers said.

It's ok to say a bit about why you sing the song, but keep it short, e.g., "This is my favorite infanticide ballad."

It's also ok to do a little proactive clarification of possible obscurities, but if the explanation is longer than the song, maybe you shouldn't be singing it.

I am annoyed by introductions that tell the story in detail. Let the song speak for itself.

Russ (Permanent GUEST and traditional singer)