This block lets you embed a python module in your flowgraph.
Code you put in this module is accessible in other blocks using the ID of this block. Example:
If you put
a = 2 def double(arg): return 2 * arg
in a Python Module Block with the ID 'stuff' you can use code like
stuff.a # evals to 2 stuff.double(3) # evals to 6
to set parameters of other blocks in your flowgraph.
Note that your custom imports will occur at the very beginning of the flowgraph's Python script, along with the other imports, so the module won't know about any parameters passed in (e.g. via command line args).