meaning of set low/set high
In the root of the expression, it is "set the bit high/low (=1/0)"

It usually means set the corresponding pin "high logic level" and "low logic level", accordingly. 

You would have to consult the documentation of your microcontroller, but it almost always is "set the output voltage to Vss/Vdd" (VssIO/VddIO if your micro has separate IO and core power supplies).