What is the meaning "Cause I laugh like I've got gold mines / Diggin' in my own back yard

This line from Maya Angelou's Still I Rise uses figurative language to convey an unbreakable positive attitude. The line specifically means that she has the happiness of someone with a Gold Mine in their own backyard.

The speaker in the poem has a tremendous amount of wealth in her own backyard, and does not need to leave home for her wealth. Maya has a powerful inner spirit, her gold mine is within, and thus no one can affect the happiness that she feels. It is a metaphor for self reliance and a happiness that comes from within.