Notification shows as number of activity done by your friends either on your post or tagged post or Friends added new post or other Activity done by your friends.
Notifications are updates about activity on website or app. You can't turn off notifications entirely, but you can go to your notifications settings to change what you're notified about and how you're notified.
1. on Top header right side you will see own profile picture and your name and there is small down arrow icon . Click on down arrow icon, opens a list of Pages. Click on "Setting".
2. Account Setting opens -> Here you can manage Notifications setting