In human families certain character may skip a generation or two and reappear. why does it happen

This is according to Mendel's law of dominance. If a parent who is homozygous dominant for a trait marries a homozygous recessive person, then all of their children will express the dominant allele’s phenotype. These children would be carriers and when marrying another carrier or a homozygous recessive person, the recessive phenotype could reappear.