Without a password, your Wi-Fi network is accessible to anybody. A strong password prevents others from connecting to your network. For recommendations on creating a strong password, see this article.
Router interfaces differ depending on the manufacturer, specific model, and firmware version. To understand router settings, refer to the user manual for your model. The user manual is typically included with the router; alternatively, you can download it from the device manufacturer's website.
The following is a configuration example for the TP-Link TL-WR841N router.