You see this bell because you are deposited as a substitute. This delegation feature can be helpful in case of vacation or illness substitutions because 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?
There are different ways to see the bell. For example, if you are not deposited as a representative of another person, but someone else represents you, then it looks like this:
If you are deposited as a substitute with another person and at the same time have deposited substitutes for yourself, the whole thing looks like this: