The bell in the top bar shows your unread count and opens a panel with Inbox, Mentions, All and Archived tabs. Clicking a notification marks it read and takes you to the thing it is about.

• Community. Replies, comment threads and reaction milestones.
• Mentions. Anywhere someone typed @ and your name.
• Classes, Courses, Coaching. Reminders, changes and new material.
• Members, Apps, Billing, System. Membership, access and account notices.

Choose which categories notify you In-App and by Email in Settings → Notifications.