Last year I received a one-year retirement visa AND a multiple entry permit.

Next month I plan to leave Thailand for the first time since I got the visa and permit. As I understand it, the entry permit allows me to leave the country as many times as I want during the one-year retirement visa/entry permit period. I also understand that I do not have to submit any other forms or make any other notifications when I leave and return to Thailand? In other words, Immigration at the airport will simply stamp my passport when I leave and return?

I just need to verify if my understanding of this is correct as I have read some conflicting posts/information online (that other forms/notifications have to be made in this regard). Thanks.

You've got the idea correct. When you complete the TM.6 -- the "arrival card" they give you on the flight -- be sure to put your re-entry permit number on it.

And, before you step too far from the Immigrations officer upon arrival back in Thailand, take a look at the date on the stamp s/he just entered. Mistakes are always possible, but much easier to correct on the spot.