Sentence:  A Unicode code unit is a 16-bit <WSD>char</WSD> value. For example, imagine a String that contains the letters "abc" followed by the Deseret LONG I, which is represented with two char values. That string contains four characters, four code points, but five code units. 
Meaning of the word marked with <WSD> tags:  A character (text element such as a letter or symbol).