Voicing has to do with the tone of the piano.  This is not to be confused with the tuning of the piano although tuning and voicing do go hand in hand. Some people prefer a bright sounding piano while others prefer a more mellow tone.  Either way, the piano's tone should be consistent.  There should not be any notes that stick out brighter or louder than the others. 

Matt Shoemaker
Piano Tuning and Repair