The options available under the text box are configurable and can be switched on or off by your administrator. This can be done through the team's admin center.
Using @mentions is a way of notifying an individual or team of a message (shown in the following screenshot). When mentioned, the individual or team will receive a notification in their Microsoft Teams client. Using the Reply button will keep messages in the same conversation thread:
A channel appears in bold if there are new, unread conversations in the channel:
