Mark explains the Pharisees’ practices of ritual cleansing (ceremonial washing for religious purity) for his non-Jewish (Gentile) audience.
The traditions of the elders were not yet written down at that time. The Pharisees believed these teachings had been given to Moses on Mount Sinai along with the written law.
These traditions were later written down around AD 200 in the Mishnah (a collection of Jewish teachings and laws). The Mishnah is one of the main sources for understanding ancient Judaism.