The etymology of the phrase is traced back to Aurthurian Lore.
Huckleberry Garlands were said to be given to Knights of the Kingdom for coming to the service of a damsel. They would approach the lady, lower their lance, and receive the small branch as a symbol of gratitude; much like a medal.
Therefore, "
I'm Your Huckleberry" literally means "I'm your Hero."
In current adaptaions, in reference to the movie Tombstone, it means "I'm your man." as an affirmative response to a challenge.
"
RINGO:
Wretched slugs, don't any of you
Have the guts to play for blood?
O.S. VOICE:
I'm your huckleberry.
Ringo turns. Doc stands there, smiling that
Cheshire catsmile.
DOC
That's just my game.
"
Taken form the offical script from the movie Tombstone.