First of all, copies mean how many copies of each data block, copies = N ways replication, 2 or more copies will support high availability.
To support high availability, user may set copies=2, the product is true software defined and use dynamic blocks, not like raid1 simply replicate volume between nodes. After a node failed and become alive, the recovery will be automatically started to fix up out of date objects.
copies=1 means there is only one data replication path means each block was placed either on node1 or node2 …
If you set copies=2 when creating storage pool, data will be replicated between two or more nodes, it will use twice space of volumes.