Diversify means to vary in type. It's often used to discuss risk in financial activities. You might diversify your investments by spreading your wealth among different types of stocks.
You may have noticed that diversify looks like the word diverse, an adjective that describes showing variety and difference. If you want to diversify your interests, that means you want to mix it up and do more than just play Dungeons and Dragons all the time — you might become active in sports, theater, and the math club. Diversify can also describe expanding into new areas. A company might diversify by sending its sales force into Asia and Latin America.