What is the zero day bug in software and apps?

We are listening this word mostly in android and windows operating systems. Developers making so many tests when they are making phone apps, computer software and operating systems. After finalization, if it is well then they will release it into the market and we will use them.

But suddenly a hacker or a security organization identified the security problem in that software by accidentally or intentionally. This is called 0-day vulnerability. This security problem is not known to anybody in the world except the hacker.

This security problem is not known to developers also. So this 0-day bugs are very dangerous.So many hackers will take an advantage of it, if it is known outside. The company will recognize the problem in their app after some loss. Then they will release the software update. Since then they are dangerous.