Quick meaning
The red number shown on an app icon indicating how many unread notifications are waiting.
Think of it like this
Like the number on a mailbox showing how many letters are inside.
Where you may see this
You may see this inside app stores, app settings, software updates, and install instructions.
Why it matters
Knowing the term helps you understand what an app is asking you to do before you tap, install, or change anything.
Say it simply
Notification Badge means: The red number shown on an app icon indicating how many unread notifications are waiting.