CpG sites are signifigance in epigentics because they are used to to regulate transcription of genes. By silence or unsilence genes using methylation. Genes that are silenced are methlylated and genes that are expressed are un methlated.
CpG sites refers to regions in the genome containing cytosine followed by guanine. cytosine is considered to be the most important nucleotide in epigenetics because it is where the methyl group attached to silence the gene.