The modbus module is exactly what you would think it is. It's a module that talks Modbus. Right now the only part of the protocol that is implemented is a subset of the function codes for an RTU Master. The configuration file is a Lua script like the rest of the modules in OpenDAX.
The module creates a thread for each port that is configured. Each port can contain any number of commands. Each command corresponds to a Modbus polling message. When the command is configured a tag is created to hold the data that is retrieved for that command. This command can be shared between different commands and both the tagname and index are given so that multiple commands can write into the same tag at different indexes within the array. For commands that retrieve coils or discrete inputs the tag will be BOOL, for tags that retrieve words the commands will be UINT.
As the modbus module starts to mature I'll spend some more time on this documentation, for now if you really need to use the modbus module for something please feel free to send me an email and I'll be happy to help. Also take a look at the modbus.conf.sample file that is in the etc/ directory in the distribution.