The Jews, like other Orientals, used their fingers when taking food, and therefore washed their hands before doing so, for the sake of cleanliness.
The reference to unwashed hands in Mark 7:1-9 is about the ablutions prescribed by tradition, according to which…
“the disciples ought to have gone down to the side of the lake, washed their hands thoroughly, rubbing the fist of one hand in the hollow of the other, then placed the ten finger-tips together, holding the hands up, so that any surplus water might flow down to the elbow, and thence to the ground.”
To neglect to do this had come to be regarded as a great sin, a sin equal to the breach of any of the ten commandments. Moses had commanded washings oft, but always for some definite cause; but the Jews multiplied the legal observance till they formed a large body of precepts. To such precepts about ceremonial washing Mark here refers.