It will show you from what browsers and devices you’ve accessed it, when and from what IP. If there’s something that you don’t recognize there or an old session from a friend’s computer, you can choose to terminate it.
Same option is available for many other online accounts, such as Facebook, Yahoo or Dropbox. Access it to monitor where your accounts have been used and end any sessions that you don’t recognize.
If you also activate two-factor authentication, your accounts will be more secure against intruders.
Credit: @Daily Security Tip