What does John 12:4 mean?

Jesus is being treated to a dinner celebrating His resurrection of Lazarus (John 12:1–3). During the dinner, Lazarus' sister Mary performed an act of profoundly intimate, sacrificial worship. She anointed Jesus with an expensive oil, which was probably most of her life savings. In doing so, she also wiped His feet with her hair, something her culture would have considered intensely submissive, intimate, and humble (1 Corinthians 11:15; John 13:1–5). The extravagant use of the oil created an aroma that permeated the entire house, leaving little doubt about the depth of her sacrifice.

Judas is pointedly labelled as the one who will eventually betray Jesus to His enemies (Matthew 10:4; Mark 3:19). John's purpose in making this point is probably to emphasize just how insincere this criticism is. According to the following verse, Judas will point out that Mary's act took up a great deal of money, which could have been used for the poor (John 12:6). One denarius was about the typical day's wages. This quantity of oil is worth 300 denarii—nearly a year's pay (John 12:5)—and was used all at once, to anoint one man! To some people in the room, Judas' complaint makes sense, and Mark explains how others echo it (Mark 14:3–11).

However, Judas' analysis is a well-dressed lie. His real anger is in seeing money "wasted" on Jesus instead of going into the disciple's communal money-bag, where he can steal some of it (John 12:6). Jesus, for His part, will defend her actions in light of His impending death (John 12:7–8).