These are the definitions for all the terms on the Audience Metrics page:
Audience: Total number of users expected to participate in the pulse.
Participants: Unique users who engaged with the platform in any way - 1) left a response, 2) voted on the open-ended responses, 3) viewed the aggregated results or any combination of those actions.
Respondents: Participants who responded to at least one question in the pulse (can be a metric or open-ended question).
Voters: Participants who voted at least once.
Participation Rate: Participants / Audience
Response Rate: Respondents / Audience
Vote Rate: Voters / Audience