If you have Multifactor Authentication configured on Office 365, when you want to use Outlook or Skype for business client, user need to insert that password and not his corporate password to use it.
Below I show you how to obtain App Password.
Log in O365 with your corporate user account and push on Settings -> Office 365.
Push on Security and privacy section -> Additional Security Check
Push on Update the phone numbers you use for account security.
On the new window, go to App Password section and push on Create button.
On the windows indicate a reference and push on Next.
Will appear the App Password. Copy it an push on close button.
Whe ypou configure an Outlook profile or want to access to Skype for business, paste that App Password.