Look at your Spaces list to see which ones appear in white bold; this tells you that you have new messages waiting. Just select one of those spaces to read your messages.
When a new message comes in, you'll also get a notification, unless you’ve turned off your notifications either in the app or in your phone settings. See Webex Teams | Tips for Managing Notifications to help make sure you don't miss any important messages.
After you read the message, the space name turns light grey in your Spaces list, letting you know that you're now up-to-date in that space.
You can also filter your Spaces list to only include unread spaces. For more information, see Webex Teams | Filter Your Spaces and Content.