Though WhatsApp makes your online status visible to everyone by default, you can easily change it. In fact, because WhatsApp has been known in the past for a variety of security and privacy issues, you may want to minimize your online footprint in the app.
Last seen and online tell you the last time your contacts used WhatsApp, or if they're online. If a contact is online, they have WhatsApp open in the foreground on their device and are connected to the Internet.
