The secondary structure of a protein refers to:

The primary structure is the linear sequence of amino acids.The secondary structure refers to the local folding of the polypeptide chain into specific shapes like $\alpha$ -helix or $\beta$ -pleated sheets, stabilized by hydrogen bonding between the amide $N-H$ and carbonyl $C=O$ groups of the peptide backbone.Tertiary structure refers to the overall $3\text{D}$ folding.Quaternary structure refers to the arrangement of multiple polypeptide subunits.