A module is a piece of software or hardware which is independent in some way.
A bus is a shared medium of communication[1]. Often, busses are time-division multiplexed[2], so if you want to use the bus, but another module is already using it, you need to wait.
For example, if your audio buffers are ultimately submitted to a sound card over a PCI bus, the submission may need to wait for any ongoing transactions on the PCI bus, such as messages to a graphics card.
A bus is a shared medium of communication[1]. Often, busses are time-division multiplexed[2], so if you want to use the bus, but another module is already using it, you need to wait.
For example, if your audio buffers are ultimately submitted to a sound card over a PCI bus, the submission may need to wait for any ongoing transactions on the PCI bus, such as messages to a graphics card.
[1]: https://en.wikipedia.org/wiki/Bus_(computing)
[2]: https://en.wikipedia.org/wiki/Time-division_multiplexing