This is a simile that means the scribes and Pharisees may appear to be pure on the outside, but they are wicked on the inside. (See: [[rc://en/ta/man/jit/figs-simile]])
"tombs that someone has painted white." The Jews would paint tombs white so that people would easily see them and avoid touching them. Touching a tomb would make a person ceremonially unclean. (See: [[rc://en/ta/man/jit/figs-explicit]])