The multi-selection feature is a convenient tool which allows you to select multiple events and tasks, one at a time. In order to select the events, you just need to tap on the colored square(s) or icons next to your event titles. You will notice, that a square/icon will turn into a grey checkbox and a counter on the top will appear. This way you can continue selecting multiple events in the future and the past. If you selected more than one item, you will be able to delete all at once or you could share them with something or somebody. To quit the multi-selection please hit the checkmark-button at the top left or deselect every single item.
Hint: If you selected an event, you can activate the date range feature in the dropdown menu (first icon to the right). By doing so, you will be able to decide about a period of time, in which all events will be marked as selected. You don't have to manually do it anymore!