It means, generally, that ${sysconfdir}/maildrop contains the full pathname to maildrop. From the courier(8) man page:

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maildrop This file contains one line whose contents is a pathname to the maildrop mail delivery agent. If Courier knows that the delivery agent used to delivery mail locally is mail drop, then certain delivery optimizations are possible. This configuration file does NOT actually specify that maildrop should be used as a local mail delivery agent, it only specifies where maildrop is installed. The default local mail delivery instructions are specified in the courierd configuration file. If the specified delivery instruction specify running an external program whose pathname matches the one specified by this configuration file, Courier assumes that it's maildrop, and will use maildrop-specific options to optimize mail delivery.

==

The default contents of ${sysconfdir}/maildrop are ${bindir}/maildrop, or:

/usr/lib/courier/bin/maildrop

Which means that if in .courier you have:

| /usr/lib/courier/bin/maildrop

Then Courier knows that you're running the maildrop delivery agent, and will supply additional options, behing the scenes, on the command line, which optimize a few things.

Normally, delivery agents need to be piped through preline, in order to add the Delivered-To: and Return-Path: headers. This usually involves an intermediate pipe and a temporary file.

If all these dominoes fall in the right order, the need for an intermediate pipe and a temporary file is removed, and the message will be delivered directly from the spool.