If your system administrator has enabled Q&A or Moderated Chat, you will see comments as they come in. You can decide which comments to make public to other viewers, and which to keep hidden. This helps keep the conversation of a livestream on track, and prevents harassment or slurs being used in your comments section.
In Q&A mode, questions and comments can only be seen by the questioner and the operator. The presenter can then choose which question to answer, without the rest of the audience being able to see the comments in the queue.
In Moderated Chat, questions and comments are again only visible to the original questioner and the operator, but the operator can choose to publish comments, making them visible to the rest of the audience of the live stream.