Head to the Chat button on the left hand side menu.
You can see what the players are saying within Minecraft chat, you can also type something within the box, which will send a message within the Public Minecraft Chat.
noticeThis will run the /say command in Minecraft, which is commonly used within the Console Page.
Minecraft guide
How to use Minecraft Chat from your Server Panel
This guide will cover the Minecraft Chat and how it can be used to chat with your players on a Minecraft server.