In a DDoS attack, your DMZ, webserver, and other internet facing servers may be hit with a flood of internet traffic, which will stop any other traffic getting in. Even if you are not directly attacked, you may also be affected by another server nearby that is the target of the DDoS attack.
To the outside world, it will look like your website or other corporate service is offline.
Think of it like a street protest against a single shop. That street protest can stop all shops in that street from being accessed, not just the shop being targeted. It also slows down any other traffic trying to go through that street to other destinations far and wide and have a knock-on effect to other streets used to go around the protest.
Note that DDoS attacks are sometimes used to distract the organisation and security teams from another more serious attack.