Please explain the teaching concerning the return of the 12th Imam.

Galveston,

First off you must understand that the belief of the 12th Imam is more highlighted in Shi'ite beliefs.
First link explains the divide between Sunni and Shia,its origin and beliefs.
http://www.islamfortoday.com/shia.htm

Here is the Sunni stand regarding the issue.
Islam Question and Answer - The status of the imams of the Ithna 'Ashari Shi'ah

Related information,as sunni muslims await the emergence of a "Mahdi",but the beliefs regarding him are different between the two sects.
Islam Question and Answer - Is the Mahdi real or not?

Hope the information sheds some light on it.

I apologise as I am unable to give a personal view because I am not well versed in Shia beliefs.

https://www.askmehelpdesk.com/islam/arrival-imam-mehdi-548436.html

If you search from YouTube about mahdi, dajjal etc. you can get a better picture of the reality, in shaa Allaah. There`s some OK material, too.