Great post explaining the step by step how to enable Multiple Factor Authentication (MFA) on all users in Office365.
MFA helps secure user sign-ins for cloud services beyond just a single password. With MFA for Office 365, users are required to acknowledge a phone call, text message, or app notification on their smart phones after correctly entering their passwords. They can sign in only after this second authentication factor has been satisfied.
A form of multi-factor authentication is included with Office 365, but you can also purchase Azure Multi-Factor authentication that includes extended functionality. For more information see feature comparison of Azure Multi-Factor Authentication versions.
Read more at source: http://blogs.technet.com/b/office365/archive/2015/08/26/powershell-enable-enforce-multifactor-authentication-for-all-bulk-users-in-office-365.aspx#.Vd5N_zZRHIW