What does it display ? What can I do ?
The task details screen let you see more information about your task such as its comments and contacts linked. It also gives you access to more actions. Here is what you can do:

1. Back to events list
Simply go back to the previous screen : events tab.
2. Contacts
You can see the participants list in details with all the contacts attending the event with their account and 2 buttons to:
Call the contacts
Send an email to the contacts
3. Done (and more)
Press this button to mark the task as done. If you're using Enterprise edition this button can open a menu with different choices, letting you access different options.
This is it! That's all you need to know about this screen.