The Full Form of SCK is‍ Serial Clock.

A slow slave device can do a clock stretching to pause a transaction by holding the SCL line LOW. The transaction cannot continue until the line is released HIGH again. On the byte level, a device may be able to receive bytes of data at a fast rate but needs more time to store a received byte or prepare another byte to be transmitted. Slaves can then hold the SCL line LOW after reception and acknowledgment of a byte to force the master into a wait state until the slave is ready for the next byte transfer in a type of handshake procedure. On the bit level, a device (either master or slave) can slow down the bus clock by extending each clock LOW period to match its bit-processing capabilities. This essentially means that even a slow device can be attached to a fast device, but the dynamics of the wired and logic will allow the transaction to occur at that lowest speed among the device connected.