You know what NFS is? Somewhere on the network there is a server, which exports a file system over the network, using the NFS protocol.
On your machine, there is an NFS client, which can mount NFS-exported file systems somewhere. That mounting can be done by hand (either by issuing a mount command, or by adding them to /etc/fstab), or one can use an auto mounter. The nice thing about using an auto mounter is that if servers go up and down (or the clients disconnects and reconnects from the network), one doesn't have to manually mount things. That auto mounter has a configuration file, and that is the "man 5 auto_master" that you mentioned above.
Is auto mounting NFS file systems a good or bad idea? Depends on your use case. If you need a certain NFS file system to always be there, you should probably put it in /etc/fstab, and hang if it is not available. On the other hand, if the computer is useful even without a file system, and the network might be intermittent (for example a laptop that might be out of range of the WiFi occasionally), you want auto mounting. YMMV.
