U.S. copyright law is intended to foster artistic and intellectual endeavors by protecting creators' rights and thus their livelihoods. However, copyright law can easily turn into a stifling intellectual monopoly (some MP3 fans would argue that this has already happened). Thus, the law contains a lot of compromises to try to keep it fair to other creators and to works' users.
The "merger doctrine" is one such compromise aimed at keeping creators from claiming unfair rights.
One of the basic tenets of copyright law is that you can't copyright ideas (industrially-useful ideas can be protected by patents, but that's a whole different subject), just the original expression of those ideas. This is codified in Section 102(b) of The Copyright Act:
In no case does copyright protection for an original work of authorship extend to any idea, procedure, process, system, method of operation, concept, principle, or discovery, regardless of the form in which it is described, explained, illustrated, or embodied in such work.
But it seems like someone could get around this by copyrighting, say, a dozen different little ways of expressing the same simple idea so that nobody else could feasibly use that idea, doesn't it? That's where the merger doctrine comes in: if there are a limited number of ways that somebody can express an idea, then you can't copyright expressions of that idea at all. This is to keep the idea itself free for everyone to use.
For instance, you can't copyright the sentence "Fred spoke." Sure, you could write it "Fred made an announcement" or "Fred shouted" or the all-time favorite "Fred ejaculated", but when you come down to it, it's expressing the simple idea of a man speaking. There are only so many ways to express that. (Whatever Fred says can of course be copyrighted if it otherwise meets the not-too-difficult criteria of original expression, of course)
The merger doctrine was upheld in a case called Toro Co. v. R&R Products Co. in 1986. The 8th Circuit Court ruled agreed that the doctrine is important in keeping a creator from monopolizing an idea. Most courts since then have ruled that the doctrine prevents an author from copyrighting a passage in the first place. However, some have ruled that the creator can copyright the passage, but that others will only be guilty of infringement if they use the exact same expression (many people don't realize that they can normally be found in violation of copyright even if they change the wording of a passage somewhat).