In the US, there are multiple kinds of intellectual property, and they're covered by multiple kinds of laws.
In particular, copyrights, trademarks, and patents are all different, and different laws apply to them.
Since the early days of the Internet, people have been spreading well-meaning but false information online about how intellectual property laws work in the US, most often because they've been told something about how one kind of IP laws work and they don't realize that things are different for other kinds of IP.
One of the most common such misunderstandings is the notion that copyrights have to be defended or they will be lost.
It's just not true. Most likely, people who say that are thinking of trademarks, where something similar is true.
But copyrights and trademarks are different things, and they apply to different kinds of IP. (Though there are some overlaps.)
In short: if you fail to defend your copyright, you don't lose it. The law doesn't work that way.
Confusion and misunderstanding about copyright are inevitable; it's a complicated topic, with a lot of gray areas. If you're interested in more information about how copyright works in the US, here are some good sources:
- A good explanation of and rebuttal to some of the most common misunderstandings about copyright is Brad Templeton's famous "10 Big Myths about copyright explained." The "defend it or lose it" myth is #5 on the list.
- The US Copyright Office provides several useful documents about copyright, including Copyright Basics (PDF) and a copyright FAQ. Neither of them mention having to defend your copyright to keep it.
- Bitlaw has a bunch of other good copyright information.
- Wikipedia's article on US copyright law is pretty good, but as always with Wikipedia, before you rely on its information (especially its legal information), double-check with a source written by a professional.