See this passage explained in the notes at Matthew 26:1-16.
And of unleavened bread - So called because at that feast no other bread was used but that which had been made without leaven or yeast.
By craft - By subtlety (Matthew); that is, by some secret plan that would secure possession of him without exciting the opposition of the people.
Ointment - This word does not convey quite the proper meaning. This was a perfume. It was used only to give a pleasant odor, and was liquid.
Of spikenard - The “nard,” from which this perfume was made, is a plant of the East Indies, with a small, slender stalk, and a heavy, thick root. The best perfume is obtained from the root, though the stalk and fruit are used for that purpose.
And she brake the box - This may mean no more than that she broke the “seal” of the box, so that it could be poured out. Boxes of perfumes are often sealed or made fast with wax, to prevent the perfume from escaping. It was not likely that she would break the box itself when it was unnecessary, and when the unguent, being liquid, would have been wasted; nor from a broken box or vial could she easily have “poured it” on his head.
Three hundred pence - About forty dollars (or 9 British pounds). See the notes at Matthew 26:7.
She hath done what she could - She has showed the highest attachment in her power; and it was, as it is now, a sufficient argument against there being any “real” waste, that it was done for the honor of Christ. See this passage explained in the notes at Matthew 26:1-16.