in host to device communication protocol, what does high and low mean?
By releasing the clock, he means that the pin for the shared clock line is placed in high impudence mode by the host. This means the host does not control or otherwise effect the line, letting the device take over. 

Since this is an open collector protocol, 'high' really means the host or slave let go of the line, letting the external pull-up resistor take over. 'Low' means they actively pull the line to ground. At no point should the host or peripheral pull the line actively high through their own pins.

in essence 'high' and high-z are the same thing here. 0 and 1 still apply as low and high.