Sentence:  There are <WSD>key</WSD> controls that adjust the “slice level” or the level at which the <WSD>key</WSD> kicks-in and starts cutting a hole for the “fill” ... Chroma key is another form of keying, which derives the key cutter or hole from a selected color. 
Meaning of the word marked with <WSD> tags:  A color to be masked or made transparent.