If I have chickens and get more eggs than I would ever want to eat, each additional egg provides me less and less marginal utility. If you have cows and get more dairy than you would ever want, then each cup of milk provided you less and less marginal utility.

If I trade you a dozen eggs that have near zero marginal utility to me but lots to you, and in return you give me a gallon of milk that has near zero marginal utility to you but lots to me, then we are both enriched by this trade. That enrichment is wealth creation.