A certain poor widow - A widow miserably poor; this is the proper import of πενιχραν, and her being miserably poor heightened the merit of the action.
Two mites - Which Mark says, Mark 12:42, make a farthing or quadrans, the fourth part of an As, or penny, as we term it. In Plutarch's time we find the smallest piece of brass coin in use among the Romans was the quadrans, but it appears that a smaller piece of money was in circulation among the Jews in our Lord's time, called here, and in Mark, Mark 12:42, a lepton, i.e. small, diminished, from λειπω, I fail. In ancient times our penny used to be marked with a deep indented cross, dividing the piece into four equal parts, which, when broken in two, made the half-penny, and, when broken into four, made the fourthing, what we have corrupted into farthing. Probably the Roman quadrans was divided in this way for the convenience of the poor. Our term mite seems to have been taken from the animal called by that name; for as that appeared to our ancestors to be the smallest of all animals, so this being the smallest of all coins was called by its name. Junius says that mite was a small base coin among the Dutch. Our word mite seems to be a contraction of the Latin minutum, a small thing, whence the French miete, a crumb, a very small morsel. See the note on Mark 12:41.

And he saw also a certain poor widow,.... Whom he took particular notice of above all the rest: the poor, and the widow, are regarded by him, and are his care; nor are their mean services, done in faith, and from a principle of love, despised by him, but preferred to the greater services of others, where faith and love are wanting:
casting in thither two mites; the value of a farthing. The Persic version renders it, "two bottoms of yarn"; See Gill on Mark 12:42,

two mites--"which make a farthing" (Mark 12:42), the smallest Jewish coin. "She might have kept one" [BENGEL].