[Android Central Question]

WiFi is on buy when I try to use an app, it says internet is offline.

[scottbeamer]

It means your network has lost its internet connection.

WiFi just gives you access to a local wireless network. The network itself may (or may not) be connected to the internet.

The ISP is down.

[Rukbat]

Or the phone is getting what's called a limited connection - it can connect to the router, but the router doesn't connect it to the internet. That's a particular difficult situation to troubleshoot, even if someone who could do it was there.