Q: What does it mean if a submission is passed from Managing Editor to Co-editor?

I would have been able to answer your question better if you had provided some more contextual information. Based on what I understand from your question, your submission was initially with the Managing Editor. Subsequently, it was passed on to the Co-editor. I think the Managing Editor is responsible for all papers within a specific subject area. The Managing Editor initially receives all manuscripts belonging to that subject area and then he/she assigns each manuscript to a specific Co-editor (who seems to be the equivalent of an Associate Editor). The Co-editor will, I presume, be your point of contact and will be responsible for processing your manuscript from end to end. 

Related reading:

• Behind the scenes: A journal editor's insight into what happens post manuscript submission
• Behind the scenes: How a journal editor reaches a decision on a manuscript after peer review