Liveness means something is happening in real-time, right at this very moment. It's like watching a video of a concert versus actually being at the concert. Being at the concert is live because you can see and hear everything happening as it's happening.
Now, imagine you are watching a video of a comedian doing a stand-up routine, but the video is actually the recorded version of a show that took place months ago. This video isn't live because it was recorded a long time ago, and you're not actually seeing the comedian perform in real-time.
Liveness is important for things like sports games and concerts because people want to experience the action as it's happening, not watch a recording of it later. We use the term liveness to describe things that are happening right in front of us, right now, so we can feel like we're a part of the action.