Here "seed" represents a person's descendants. Alternate translation: "your descendants will be many" (See: [[rc://en/ta/man/translate/figs-metaphor]])
Here "offspring" are spoken of as if they were as many as blades of grass, and probably as thriving, too. Alternate translation: "your descendants will be as many and as alive as the grass that grows" (See: [[rc://en/ta/man/translate/figs-simile]])