Once login notifications have been turned on, you'll receive them whenever someone accesses your account. The login will pop up on your device or in your email inbox, depending on which settings you chose. At that point, you can tap on "This was me" if the login was you, and no further action is needed. If you don't know who logged in, press the "This wasn't me" button, and Facebook will guide you through the password reset process to protect your account.