What are the functions of disulphide bonds?
Disulphide bonds occur in proteins, not amino acids, although they involve a covalent bond between two amino acids (both cysteine).

The received wisdom is that disulphides are used as extra stabilisation of the structure of proteins which are secreted, or which have an extracellular domain. It's important to get hold of the idea that they do not occur in the cytoplasm, but are only formed in proteins which pass through the secretory pathway: they are formed in the endoplasmic reticulum (ER), and there are resident ER proteins (protein disulphide isomerases) whose function is to assist in their formation.

Classical examples of disulphide-bonded proteins are insulin (where the A and B chains are linked by disulphides) and immunoglobulin G where each Ig domain has an intradomain disulphide, and where the heavy chains are linked by disulphides. There are, of course, many others.

Cells also contain the tripeptide glutathione which includes a cysteine residue. the ratio of oxidised (disulphide bonded dimers) to reduced (monomers) glutathione helps to set the redox state of the cytoplasm and the lumen of the endoplasmic reticulum.