Here is one of my examples with 6 activities and 1 silent step: 
+  ( 'Ac1', ->  ( 'Ac2', 'Ac3' ) ) )
tells that you should execute the Process Tree Ac2->Ac3 before executing the Process Tree Ac1. In true concurrency, you can execute Ac1. So the possible traces are Ac1->Ac2->Ac3 and Ac1->Ac2->Ac3->Ac1.
Note: in pm4py objects/process_3tree.obj.structures.process_3_tree.structures.node is defined to hold information about a process tree node. One of its properties is parent  (the parent node, which is left empty for the root node)