As far as I know, selenocysteine is the only reason you need selenium in your diet (which you almost certainly get enough of; the requirement is vanishingly small, <100 micrograms/day). It is a member of the same group as oxygen. As you go down a group, things change in subtle ways. Sulfur is a less-electronegative, bigger, more polarizable, more nucleophilic, stinkier oxygen. Similarly, selenium is all these things, but more so!
It is the nucleophilicity that is so important; it is found in the amino acid selenocysteine – that is, the selenium analogue of the sulfur amino acid cysteine (or the oxygen amino acid serine).
Selenocysteine plays a role in thioredoxin reductase, the reaction of which with curcumin may have chemotherapeutic applications.