Albert Barnes Commentary Matthew 26:13

"Verily I say unto you, Wheresoever this gospel shall be preached in the whole world, that also which this woman hath done shall be spoken of for a memorial of her." — Matthew 26:13 (ASV)

A memorial. Anything to produce remembrance. This should be told to her honour and credit, as a memorial of her piety and self-denial; and it is right that the good deeds of the pious should be recorded and kept in remembrance.