If the front end is the shop floor, the back end is the warehouse, the logistics and the accounting. It is where data gets stored, rules get enforced, payments get processed and accounts get managed. Users never see it directly; they only feel whether it works.
When you log in, the front end shows the form, but the back end checks your password. When you order, the front end shows the button, but the back end charges the card and saves the order.
Why you keep hearing it
Because it is half of every conversation about building software, and because it is the half where AI-built projects most often wobble. Generating a beautiful page is easy now. Handling real user data, securely, at scale, is still serious engineering. "The back end" is also where developers locate many problems: slow back end, back end is down, we need a back-end fix.
What it means for you
Mostly vocabulary, but with one useful instinct attached: when evaluating an impressive AI-built demo, ask what happens behind it. Where does the data go? What happens with a thousand users? A demo can be all front end. A product needs both halves.