From the FAQ:

What are levels, and how can I increase my level?

Levels range from 0-10, with level 0 being the default after registration, and level 10 being the highest level attainable. Levels represent a user's contributions to the community. At their core, they are calculated based on a user's contributor value. When a user creates a giveaway that's received by the winner, their contributor value and level increases. This allows the user to access additional giveaways that are restricted to higher levels. If you mouse over your level in the navigation bar, or the level on your user profile, you'll see a tooltip with the level rounded to 2 decimal places. This will give you a better idea of small increments in level, and when you're about to level up.

Levels increase in difficulty almost exponentially. Below is a summary of levels and their corresponding contributor values.
    Level 0: $0.00
    Level 1: $0.01 - $25
    Level 2: $25 - $50
    Level 3: $50 - $100
    Level 4: $100 - $250
    Level 5: $250 - $500
    Level 6: $500 - $1,000
    Level 7: $1,000 - $2,000
    Level 8: $2,000 - $3,000
    Level 9: $3,000 - $5,000
    Level 10: $5,000+