When you discuss something in a meeting, everyone can hear what you're talking about. If you don't want everyone to hear your discussion, you can suggest talking about it "offline":
I don't want to get into that here, but we can talk about that offline.
The word "offline" comes from computers. When something is "offline" it's not connected to the Internet or to another system.
You can use "talk about ___ offline" in situations like Internet forums or email groups. Two people can discuss a topic "offline", which means discussing it directly on email or through direct messages.