Secure is a strange word because there is no one-sentence explanation that defines it well. Anything valuable that can be misused can benefit from some type of protection.
Security refers to any measures taken to protect something.
Examples of security in the real world include locks on doors, alarms in our cars, or security staff including police. While this is a broad term that could even refer to protecting a computer from physical theft, we usually use it to describe methods of protecting confidential information stored on a computer. Increasing your own knowledge is your best defense. As you learn the practical information about how computer criminals gain unauthorized access, you can employ better means of protection. Information about you including financial information and account numbers, medical history, race, and marital status is probably stored on computing equipment somewhere.