Role reversal is when the roles in the scene have reversed the normal or traditional power dynamic. In role reversal, the person who normally has the lower status takes a dominant approach and the person who normally has the higher status takes a submissive approach. This is not the same thing as switching unless the play partners usually have clearly defined D/s roles.
(The slash notation means "dominates", e.g. "punishes" or "spanks":)
In a traditional setting, the man has higher authority than the woman, and therefore all F/M has an element of role reversal attached to it, but less so in more modern relationships.