You see this bell because a coworker selected you as a delegate. This delegation feature can be helpful in case of vacation or sick leave delegations as your substitute receives notifications about all your existing tasks and appointments. This way, you can avoid essential tasks remaining undone or appointments are not attended to.
If you want to know how to enable this delegation feature, look at the following HelpCenter article: How do I activate the delegation feature?
If you are not a delegate of another person, but someone else covers for you, a click on the bell icon shows something like this:
If you stand in for another person and at the same time have some one who substitutes for yourself, it looks like this: