What does the change in 'participants' mean in the change log?

If you use the change log within your form, it is possible to see a change that indicates 'participants' were changed. This change is not related to any specific field on the form, but the actual participants within the workflow. 

The 'participants' change indicates that some of the users in the workflow have changed since it was originally launched. That may be because a value in the form changed, which updated who would be involved on a step; a user within a group had been changed; or a step that had not yet been reached in the workflow was updated and it changed the users at some point.