Privacy Mode is a feature that is only available to MAC and Android. It allows you to pixelate your screen that's being broadcast to hide sensitive information you do not want shared with your viewers. Privacy Mode can be turned on when you need to hide such things as entering your iTunes/Google Play Store password, email address, credit card information when making iTunes/Google Play Store purchases, and more.
To enable Privacy Mode while streaming, click on the icon that looks like a square with 4 different blocks on the inside of it. It's the icon that is circled after you click the camera icon on the bottom. Once you press on that and the icon is highlighted in blue, you will see a notification pop up letting you know that Privacy Mode is active and that the details of your screen are not visible to your audience.