WhatsApp is a phone app which lets you participate in group chats with your contacts. It also does voice calls and video, and it's encrypted, which means it should be impossible for WhatsApp or others to read the content of those chats and calls.
Signal is another phone app which does all those things too.
So why am I boring all my friends into swapping WhatsApp for Signal?
Well, the main reason is that WhatsApp is owned by Facebook, and they've recently made changes to make it easier for them to hoover up your contacts list and share it with Facebook. So while the messaging bit itself gives good privacy, having the app on your phone can not only compromise your own privacy, but also that of anyone whose phone number you have stored.
Signal is a pretty good functional equivalent, operated by a non-profit rather than a giant advertising business, so give it a try.