A Module is a group of blocks that have a common goal. You can use Module to make your bot well-organized. A module can also contain other modules and blocks. Below is an example of Shopping Module with 2 submodules and an Image block:
Also, bot builder can group blocks to Order and Customers' details. In case you want to copy many blocks in the flow or restructure your bot flow, it is better to create blocks in a module then duplicate that module, rather than copy the block one by one in the main flow.
At the end of the module, if you want to continue the flow with different exits, you can use Module Outlet. Then, you can connect Module Outlet 1 to a different block/ module and Module Outlet 2 to another block/ module.