A Run Id is a globally unique, platform-level identifier for a Workflow Execution.
Temporal guarantees that only one Workflow Execution with a given Workflow Id can be in an Open state at any given time. But when a Workflow Execution reaches a Closed state, it is possible to have another Workflow Execution in an Open state with the same Workflow Id. For example, a Temporal Cron Job is a chain of Workflow Executions that all have the same Workflow Id. Each Workflow Execution within the chain is considered a "Run".
A Run Id uniquely identifies a Workflow Execution even if it shares a Workflow Id with other Workflow Executions.