When you open Action Center in Windows 10, you will see Focus assist in the notification section. This feature started available since Windows 10 April 2018 update (version 1803). This feature is aimed at helping you to minimize or disable notifications temporarily during a certain period or when you are doing certain activities.
Focus assist is pretty useful in a certain circumstance. For instance, you can enable this feature when you need to stay focused or when you are making a presentation with your Windows 10 laptop. Focus assist offers you two options (called focus level) on how to minimize notifications on Windows 10: Priority only and Alarm only. If you select Priority only, Windows 10 will only display notifications from apps listed on the priority list — which by default consists of Nearby sharing, Snip & Sketch, Xbox Console Companion, and Xbox Game Bar. You can add more apps to the priority list.
If you select Alarm only, Windows 10 will not display any notification except for the alarms.
How to enable or disable Focus assist on Windows 10
There are two ways to enable or disable Focus assist on Windows 10. First, you can right-click the Action Center icon on the bottom-right corner of your screen and select Focus assist. There three options available. Whether you want to disable it, enable the Priority only option or enable the Alarm only option.

Alternatively, you can also enable/disable Focus assist via Windows Settings. To do so, open Windows Settings by clicking the gear icon on the start menu. Select System on the Windows Settings main window and click the Focus assist on the left panel. Select one of the radio buttons. Select Off to enable Focus assists, Priority only to display notifications from the apps listed on the priority list, and Alarms only to hide all notifications but alarms.

How to add apps to the priority list
If you set Focus assist to Priority only, Windows 10 will only show notifications from the apps listed on the priority list. As mentioned earlier, there are about 4 apps on this list by default. They are Nearby sharing, Snip & Sketch, Xbox Console Companion, and Xbox Game Bar. You can update this list by adding new apps. To do so, go to Focus assist on the Windows Settings (System -> Focus assist) and click the Customize your priority list link.

Go to the Apps section and click Add an app and select the app you want to add to the priority list.

To remove an app from the priority list, you can click the app you want to remove and click the Remove button.

In addition to apps, you can also add contacts to the priority list. Only contacts on this list will be displayed the notifications from. Apps that support this feature (Focus assist) are Mail, Skype, Calling and Messaging.
To add contacts to the priority list, click Add contacts on the People section.

Select the contacts you want to by clicking them. Once done, click the checklist icon on the bottom-right corner.

How to manage Focus assist automatic rules
As mentioned earlier, you can use Focus assist to temporarily disable notifications in Windows 10 for a certain time span. For instance, you might want to disable notifications during office hours from 9 AM to 5 PM each day. Or, you might want to disable notifications when you are playing a game or open an app in a full-screen mode. Focus assists allow you to accomplish them via automatic rules.
To manage automatic rules, go to Focus assist on Windows Settings (System -> Focus assist). To disable notifications during a certain time span, click the During these times option.

Set the start and end time, respectively. Also select the focus level.

To disable notifications when you are playing a game or open an app in a full-screen mode, you can simply select your option by clicking it and set the focus mode on the next window.

If you want to disable certain rules, simply slide the blue button on each rule.