Fig 1. The I2C bus, 1 MASTER , 3 SLAVEs, and the required pull-up resistors.
There are two kinds of device on the I2C bus:
MASTER - node that always controls the clock line
SLAVE - node that is not in control of the clock line
The most common setup is 1 MASTER and multiple SLAVEs on the I2C bus. Both MASTER and SLAVE can transfer data over the I2C bus, but that transfer is always controlled by the MASTER.