OAuth and mail client?

How can customers who don't speak tech jargon know whether we need to get this secure mail key thing? AT&T's message talks about e-mail clients and apps, but I don't know what clients are. And apps? I know I don't use Outlook, but may use GMail on mobile. On my PC and laptop (both Windows 10 OS), I use Firefox and just sign-in on the ATT-Yahoo mail page. On my Android phone, it's an "email" icon that may be associated with Gmail. (A lot of Google stuff came with the phone. and the store clerk who sold me the phone set it up.)

So, in plain English, how can I determine whether I need to make this change? Thank you.

--CalWriter