Chromatin accessibility refers to how easily the DNA within chromatin can be accessed by proteins and other molecules. Chromatin is a complex of DNA and proteins that helps package DNA into a compact form within the nucleus of a cell. When chromatin is tightly packed, it is less accessible, making it difficult for transcription factors and other regulatory proteins to bind to the DNA and initiate gene expression. Changes in chromatin accessibility can influence gene activity and are crucial for processes like cell differentiation and development. Techniques such as ATAC-seq and DNase-seq are used to study chromatin accessibility, helping researchers understand how genes are regulated in different cell types and conditions.