My ID, the random letters and numbers on the side, changes with pretty much each post I make and I don't know why.
I've looked up other people like Mag to see if their ID changed over the years, but they've had the exact same one since 2013 while mine changes every five seconds.
A little help, please? I don't understand what's going on.
To answer your question, your ID is an encrypted version of your IP, which is basically a number that identifies your computer and internet provider. Most people have dynamic IP's that change every x number of minutes (and when they restart their computer or modem). Mag, Astin, and some others are lucky enough to have static IP's, which means their IP doesn't change until they tell it to.
You have internal and external ID's. Your external ID is what shows next to your name; it's unique to each board and will change when your internal one changes. Your internal ID is saved in your browser cookies. So every time you restart your browser and visit the site, it will update (and will change if your IP has changed).
Your internal ID is also the password used to delete posts. You can actually check it in your cookies. If you save the ID shown there, you can switch the ID out in the browser bar after hitting Del. This will let you delete any post you made while that ID was active.