Something that's affected is influenced or acted upon. If a bottle of ointment says "apply to the affected area," you should put the medication only on the parts of your skin that touched the poison ivy.
One of the most common vocabulary mix-ups is effect and affect: effect is usually a noun, and affect is usually a verb that means "to influence" of "act upon." Affected is the adjective form of the verb. After a flood, affected homeowners might try to get insurance. A sad movie might leave you deeply affected. The word can also refer to behavior that's done only to impress someone: if you're acting affected, you might use big, fake melodramatic gestures.