How is one supposed to use passman's OTP?

Hi NC community !

I use the Passman app and couldn’t help to notice the ‘OTP’ tab when storing a new password.
I think I get the idea of what OTP is, but I can’t seem to understand how I could integrate this with external services (Facebook, Google, Skype…).
Can anyone help me understand exactly what it is and how i’m supposed to use it ?
Thx in advance !
And of course, thx to all contributors, this soft is amazing !