So, What Are “Frankincense and Myrrh”?

These two items show up all over the place in Christmas music. This verse from the modern carol “In the Silence” by Craig Courtney with words incorporating an Appalachian folk song is particularly pointed, as it includes the symbolism quite plainly:

Have you ever in the silence wondered at the thought
Of how it came to pass the Wise Men chose the gifts they brought?
Frankincense for holy ones, and gold for kings, but myrrh?
Why a gift for tombs they brought on this, the Savior’s birth?