If you have unread message(s) from a chat/conversation in your inbox then there are multiple ways to mark them as read.

The most common and simplest way to mark it as read is to click on the inbox item, this will redirect you to the conversation/chat detail (depending on what item you've clicked on).

Another way is to hover your mouse over the chat/conversation inbox item so the 3 action icons appear at the end of the row. And click on the playlist_add_check mark as read icon.

Clicking the thumb_up like icon will also result in marking the corresponding inbox item as read, alongside with letting the author of the message know that you like the idea, or you can use it instead of answers like: "ok", "cool", "will do" and so on.

The very last way to mark an inbox item as read is to check the box on the left side of the item and click the mark as read button in the New header, as shown in the screenshot. This way you can get rid of multiple items!