E: The repository 'https://packages.elementary.io/appcenter bionic Release' does not have a Release file.
N: Updating from such a repository can't be done securely, and is therefore disabled by default.
N: See apt-secure(8) manpage for repository creation and user configuration details.

^ Is a message I get on with the update manager and/or terminal command update/upgrade. Can anyone tell me what this is, if it's specific to any programs, important, and/or what to do about it?

-Much appreciation
and with kindest regards

Nothing you can do except contact the incompetent manager of the mirror and get him/her to correct the poor configuration with the correct files.

But that repository does have Release, Release.gpg, and InRelease. What it doesn't is support secure connections (HTTPS). Edit your /etc/apt/sources.list or whatever file this repo is specified in and change https to http.

Moreover, the repo only has the main section, so if that line in sources.list includes other sections (restricted, universe, whatever) then remove them.

Ah my apologies to the mirror maintainer and the the problem is solved, PEBKAC, an oldie but a goodie that continues to rear it ugly head time after time.

This doubtlessly seemed the most useful, though I appreciate seeing anything about Linux. :ap
As an intermediate newb the same way though, I'm at a loss of what any of it means.
Genuinely appreciate any and all help, the same way that maybe someone will see this and have their issue resolved the same way.

-Kindest Regards;
All Hail LT \vWMv/

He checked the mirror unlike me assuming incompetent manager.

This is what a /etc/apt/sources.list looks like it is where your error is.

This is from my rasberrypi. The lines that start with a # mean nothing they are commented out by that character and are not read for anything at all. The only line in there that is used is the deb https:// it is this part he told you there was mistake the s should not be there in your file it does not support the secure transfer protocol of http. This does not matter a whole lot as all packages are signed and verified with a gpg encryption key before the upload to mirror, once on the mirror or when downloaded on machine installed to, a very secure method of doing it. He also tells you that the mirror only has a main directory for its repository my line below.

This tells apt to use the deb source of https://mirror.herrbischoff.com/raspbian/raspbian/ the buster repository on that mirror with the enabled main contrib non-free rpi sources in that buster repository. My line edited to changes that would be required to be made according to the poster for your mirror.

You would use the command nano /etc/apt/sources.list to open the file for editing logged in as root or using sudo if setup for that method. Once done with the editing then a control plus x to exit program answer y for yes to saving the changes then apt update
if you get stuck post the file contents here and I or someone else will give it an edit.

Edit: and now I read it again the last part with the commands to use you need to have a Terminal open to use them in, I thought I had left something out and was correct in that when checking it again..

But it is very simple:
Why you have difficulty editing file with root?