Your quota in Office 365 is taken up by your email, calendar and any other information that you have stored in your Office 365 account. Queen's offers 25 GB of storage space for every users on Office 365.
It is important to be aware of your quota status because you will be unable to send email if you exceed your 25 GB limit. To clean up your data and free up space, remember to periodially delete the items you no longer need, and empty the Deleted Items folder.
To check your quota in Office 365 ensure that you have Mail selected. Hover over the name of your mailbox. The Tool Tip will inform you how much space you have used.