To access the notifications panel, click the bell symbol on the right side of the grey navigation bar. Your notifications panel displays reminders that you have set against invoices as they become overdue, and when other users have mentioned you in an invoice note. You can find out more about setting reminders on invoices here and using mentions here.
When you have an 'unseen' notification, a number will appear next to the bell symbol to indicate how many notifications you have outstanding.
You can mark notifications as 'seen' by clicking the orange bubble on the left of the notification, or by clicking 'Mark all as seen' at the bottom of the notifications panel.
Clicking directly on the text of the notification will link you to the relevant invoice page, and mark the notification as 'seen'.
Your notifications panel is specific to your user and organisation, meaning other users will not see your notifications.