When you receive a request notification (i.e. join a meeting, approval or a task) these are on activities you need to respond to. To know if you have a request, look at the top of your portal and look for the world icon (see below).
When you click on the world icon, you'll see the full list of request and be able to respond directly in the list or click the hyperlinks to travel to the associated content. See above view of the request sections.
A 'Requests' dropdown link will also appear in your top bar and a red notification will inform you of the amount of requests you have to respond to. Click on this to respond.
Requests include invitations to groups or events, as well as announcements of tasks. Your request alert will stay until it is dealt with.