CVSup is a software package for distributing and updating source trees from a master CVS repository on a remote server host. The FreeBSD sources are maintained in a CVS repository on a central development machine in California. With CVSup, FreeBSD users can easily keep their own source trees up to date.
CVSup uses the so-called "pull" model of updating. Under the pull model, each client asks the server for updates, if and when they are wanted. The server waits passively for update requests from its clients. Thus all updates are instigated by the client. The server never sends unsolicited updates. Users must either run the CVSup client manually to get an update, or they must set up a cron job to run it automatically on a regular basis.
The term "CVSup", capitalized just so, refers to the entire software package. Its main components are the client "cvsup" which runs on each user's machine, and the server "cvsupd" which runs at each of the FreeBSD mirror sites.
As you read the FreeBSD documentation and mailing lists, you may see references to sup. Sup was the predecessor to CVSup, and it served a similar purpose. CVSup is in used in much the same way as sup and, in fact, uses configuration files which are backward-compatible with sup's. Sup is no longer used in the FreeBSD project, however, because CVSup is both faster and more flexible.