The Activities tab provides a list of actions performed on your account, from users logging in to your account to administrators modifying account settings. This list is arranged chronologically, with the most recent actions appearing at the top of the list and the oldest appearing at the bottom.
From this sub-tab, you can set email alerts to notify you any time certain actions occur within your account. Alerts can be set for everything from logging in to modifying permissions, even down to alerting you every time a document is changed.
Some settings may help increase account security, such as alerting you whenever a password is changed, or simply help keep you up to date, as with the options to let you know when a document has been viewed or shared.
Once you've made your selection across the tabs, click Save to ensure you start receiving notifications.