Focus assist (also called quiet hours in earlier versions of Windows 10) allows you to avoid distracting notifications when you need to stay focused, and is set by default to activate automatically under certain conditions. Here's how to turn focus assist on or off:
Select the action center icon on the taskbar.
Select Focus assist to cycle through the available settings: either Priority Only, Alarms Only, or Off. (If you don't see the Focus assist tile, you may need to select Expand first.)
Focus assist settings can also be edited by selecting Start > Settings > System > Focus assist, or type Focus assist into the search box on the taskbar, and then select Focus assist settings from the list of results.