Since I've never been very good at math, determining resolution has always made me nervous. But if you use Photoshop, you have to know it. And it starts with understanding pixels.
A pixel is a tiny square, and the term is short for "picture element". The whole idea of pixels is that they're so small that when you look at an image, it all blends together in your eye. If the pixels aren't small enough, you'll see the squares, like in old '80s video game images. The reason that pixels were so much bigger back then is because it takes a LOT of computing power to deal with little pixels. I'm no computer nerd, but it's mind-boggling for me to think how much computer power it takes to play a high-definition movie nowadays, as opposed to just showing a bunch of big pixels on an old-time video game.
Like I say, I'm not comfortable around math, so I just go with the specs (specifications) given to me. Nowadays I mostly post images on the web, and I've gone with the standard that Google gives that allows you to upload an unlimited amount of images for free, which is 2048. When someone sends me a file that's 2048, I know that they're doing the same thing. Up until a few years ago it was 1600, which seems kinda small now. 2048 will work for an image even on a nice big desktop computer, like the one I'm on now (an iMac) and it's more than enough for a tablet, or a phone.
2048 is the number of pixels wide that an image is. I use Photoshop, and check it under Image > Image size. It simply means that if you counted the number of pixels across the widest part of the image, it would add up to 2,048 pixels. I consider that to be standard resolution. When you share photos, or email them, that's plenty of resolution. anything larger than that is a waste of resolution, and smaller than that can cause the images to appear blurry, or "pixelated".
But sometimes I like to see more. So I use the term super high-resolution. And that means an image that's usually about twice the size of standard. These files are much so large that I upload them to my server, and provide a link so that people can see them from there. They usually fill up the whole screen, and require you to scroll around to see everything. I especially like old historic photos with lots of detail, which I save into a folder called SHR (Super High Resolution). These take a fair amount of time to create, in Photoshop, and then need to be FTPed (File Transfer Protocoled) to a server, and I use Dreamweaver for that. In order to do this I need, in addition to a very powerful computer, Adobe Creative Cloud, and my website, which I pay for. I share these super high-resolution images every once in a while on Facebook, but mostly I put them on my Patreon page, as a reward for my PhD supporters.
And then I have a folder called SDHR. Like most people who work on a computer, I have a childish sense of humor, and know that I have to give a name to things, and it really doesn't matter what name it is as long as I can remember it. Yeah, you guessed it, my SDHR folder is "Super Dooper High-Resolution". I really don't save much stuff to there because most images don't have enough detail to warrant the extra work it takes for me to create it. SHR is fine.
My career grew up with desktop publishing, Macintosh computers, and the web. I'm very comfortable there, and I often take it all for granted. If it all sounds like gobbledygook to you, I understand. This is my world, this is what I do. If you're not sure what to do, just sit back and enjoy!