Active /active (which really is a tern that shouldn't be used any longer) is just 2 active/passive SQL server failover instances on the same cluster.

More correctly called 'multi-instance cluster', it means you have a 2 node cluster with 2 SQL Server instances running on it (each serving their own databases), one instance runs on node 1, one runs on node 2. If either node fails, the instance running there fails over to the remaining node, resulting in 2 SQL instances running on the same node.