As I understand both comments, a ‘Word Clock Generator’ & ‘Master Clock’ are the same thing
A Word Clock Generator is a discrete “box” that generates timing signals, often in addition to word clock. Other devices “sync” to its timing signals.
A Master Clock is not a specific device, it is a function provided by some device that the other devices sync to. The device that serves as the “master clock” can be a discrete word clock generator, an audio interface, a component of a video chain, or some other source.
In order for a digital system to work properly, all of the devices in the chain must use the same clock source to avoid, to use a broad example, one device starting to count at the exact second and another starting a half second later.
Many digital devices have built-in word clocks, and can serve as a master clock, the quality of these clocks can vary. For this reason (among others), a separate word clock generator is often used in more sophisticated facilities. While most discrete word clock generators have multiple outputs for a given format, generally a single output is used to connect all of the devices that are feeding a single computer. The other outputs are typically used to feed other digital channels, such as might exist in a multi-room recording studio or post-production facilities.
Hope this helps clarify things a bit…