19th century slang which was popularized more recently by the movie Tombstone. Means "I'm the man you're looking for". Nowdays it's usually used as a response to a threat or challenge, as in the movie.
"Who thinks they can beat me?"
"I'm your huckleberry."
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.
And if anyone disagree with this information: I'll be YOUR Huckleberry!