At the dinner table, Mary used her valuable fragrant oil to anoint Jesus, presumably out of thanks for what Jesus did. However, Judas raised a question. Judas asked why the oil is not sold for 300 denarii (AKA one year of wages) to be given to the poor. Keep in mind that while the wordings may portray care for the poor, Judas focused on getting the money. Some questions have hidden motives beneath them.
Jesus responded by asking Judas to "let her alone" because she "kept [the fragrance] for the day of [His] burial" (12:7). Mary wanted to express her value for Jesus (especially with what Jesus did), and the anointing with valuables is symbolic of Mary choosing Jesus. Not to mention that Mary is likely saving some of it during the burial day, suggesting that Mary wants to express thanks while Jesus is still alive. In fact, Jesus told the disciples that Jesus would not always be here (especially since he would later die on a cross), but they would be able to have poor people (especially people in need of Jesus) with them always (12:8). Jesus (and the disciples) can always do stuff from the poor anyway.