One workflow run is an execution of all workflow steps on one task. Depending on how you workflow structured, not all steps might be executed in one run.
For example, if you imagine that you want to check text sentiment, then your workflow might have two steps - a check that a task has a PDF attachment and then an execution of A.I. model for text sentiment. Then, if task has no attachment, it will rejected by the first step (no PDF attachment) and this will count as one workflow run.