A crowd blocking a shop's entrance. It steals nothing - it just doesn't let anyone else in.
Picture a shop with one door. Customers come in, buy, leave - a normal day. Now picture a crowd gathering in front of that same door, one that doesn't want to buy anything. It just stands there and blocks the entrance. Real customers see the crush and give up. That's exactly what DDoS is, except the door is a site, and the crowd is requests - the commands your computer sends every time you open a page.
Under normal traffic, a site gets requests one after another and answers calmly. In a DDoS attack, thousands of other computers - infected, or deliberately pointed by an attacker - send requests all at once, without a pause. The machine holding the site up tries to answer all of them at the same time. It can't. It slows down, then stops altogether.
The owner of an online shop launches a big sale on a Sunday morning. Right then the site stops loading. Not because of a broken server, not because of an expired hosting plan - someone has deliberately pointed a crowd of other people's computers at the entrance, at the busiest moment. The shop stays closed exactly when it should be selling the most.
Defense doesn't fight the crowd at the door itself - it stops the surge before it ever gets there. A service sits in front of the site and checks everyone coming in: a real visitor, or part of the flood. Real ones pass through without noticing anything. The rest get stopped out front, and the site's own door never even registers that there was an attempt to storm it.
Here's the cost
Building AI systems myself, I explain this attack to clients first - it sounds frightening, and it's actually the simplest to understand. There's no hacker sitting there digging through your data. There's just a push, meant to bring you down for a few hours.
That's also why the defense here is different from everything else in the topic. Here you're not guarding a secret - you're guarding capacity. You put someone in front of the door to count the crowd for you.