Utility and consumer choice
Utility is the satisfaction a consumer gets from consuming a good. Consumers aim to get the most total utility possible from their limited income, which is why they typically spread spending across different goods rather than buying only one thing.
The law of diminishing marginal utility
As a person consumes more units of the same good in a given period, the additional satisfaction (marginal utility) from each extra unit tends to decrease -- the first unit of something is usually worth more to you than the tenth.
Example 1: A student eats plates of rice one after another. The first plate gives high satisfaction; by the fourth plate, they feel little extra enjoyment and are close to full. Explain this using the law of diminishing marginal utility.
This illustrates diminishing marginal utility -- the extra (marginal) satisfaction from each additional plate of rice falls as more plates are consumed, since the student's need/desire for rice is progressively satisfied, until the extra utility from another plate becomes very small.
Consumer equilibrium
A rational consumer with a fixed income and multiple goods to choose from reaches "consumer equilibrium" -- the spending combination that maximizes their total utility -- when the marginal utility gained per naira spent is equal across all the goods they buy. If one good gives more satisfaction per naira than another, a rational consumer would shift spending towards it until the balance evens out.