Seed balls, simply put, are a method for distributing seeds by encasing them in a mixture of clay and compost. This protects the seeds by preventing them from drying out in the sun, getting eaten by birds, or from blowing away.
Seed balls are scattered directly on the ground, not planted. Self-sufficiency and sustainability website Path To Freedom says seed balls are useful for seeding dry, thin and compacted soils and for reclaiming derelict ground (which is why they are often used in guerilla gardening). Seed balls are particularly useful in dry and arid areas where rainfall is highly unpredictable.