According KB, that semisync waits for one replica for acknowledge.

Semisynchronous replication waits for just one replica to acknowledge that it has received and logged the events.

In some usecases with multiple slaves, itcan be useful to increase the amont of server
which have to acknowledge the transaction before process, like described in

https://dev.mysql.com/doc/mysql-replication-excerpt/8.0/en/replication-options-source.html#sysvar_rpl_semi_sync_source_wait_for_replica_count