What does the slave I/O thread do?

The slave I/O thread connects to the master and requests it to send updates recorded in its binary logs. This is one of the three main threads involved in MySQL replication, along with the binary log dump thread on the master and the slave SQL thread on the slave1. The slave I/O thread is responsible for reading the updates that the master’s binary log dump thread sends and copying them to the local files that comprise the slave’s relay log1. The state of this thread is shown as Slave_IO_running in the output of SHOW SLAVE STATUS1.