A daemon is a program that always is running in the background on the system, initialized and waiting for something to do. MiniVend runs as a daemon in the normal course of events.
Other programs that you may be familiar with that run as daemons in most cases:
HTTP server (Apache, etc.)
MySQL, mSQL, Oracle, and other SQL servers
Sendmail
MiniVend takes an appreciable amount of time to compile and load even on the fastest systems. It has many configuration options, and can serve hundreds of catalogs. If it were to be loaded every time a user accessed it, it would be unusable.
The daemon approach allows a rich set of features to be accessed fast. The actual CGI program (vlink or tlink) is a small program written in C -- it communicates with the minivend daemon.
One of the side effects of this is that when you change a configuration file, you must tell MiniVend. You do this either by restarting the server, or better yet, using the reconfig script to reconfigure an individual catalog.