Silent notifications are ones that show at the bottom of your notifications panel and do not trigger a banner pop up when they activate. So you can hide these ones from your Lock Screen. To make notifications silent long press a notification and you have options. With conversations you can have them priority, default and silent. Anyway if you don’t want certain notifications popping up when using your phone change them to silent. This is where you want to see a notification but not be bothered by it.