The Pharisees are like unmarked graves because they look ceremonially clean, but they cause people around them to become unclean. (See: [[rc://en/ta/man/jit/figs-simile]])
These graves were holes dug in the ground where a dead body was buried. They did not have the white stones that people normally place over graves so that others would see them.
When the Jews walked over a grave, they would become ceremonially unclean. These unmarked graves caused them to accidentally do that. This can be stated clearly. Alternate translation: "without realizing it and become ceremonially unclean" (See: [[rc://en/ta/man/jit/figs-explicit]])