Marginal utility is the satisfaction one hopes to get when consuming an unit of a good. As total utility reaches its maximum(total fulfillment of the need that the good hopes to satisfy(, marginal utility will go to 0)the last unit of the good satisfied the need completely(. We can say that total utility and marginal utility are negatively corelated as when one increases the other decreases.
When the marginal utility reaches 0 and another unit of the good won’t bring any more satisfaction, the rational consumer will stop consuming the good, as there is no point in it. If he continues to consume it, it is not the same need that needs to be satisfied as it is already done. The need will shift to another one, satisfied by the same good and once again marginal utility will decrese with each unit consumed until the new need is fully satisfied (total utility in the case of the new need reaches 0).
As an example we can take a woman shopping for shoes, the first pair she buys will satisfy the need of protecting her feet, but if she continues to buy them, the need will shift to having shows to match her clothes.
Ilie Ioana Antonia 