In polymer Buna- S: ' S ' stands for:

Buna--$S$, $' S '$ stand for styrene.

Buna – S is a more commonly used name for a polymer named styrene butadiene rubber. The structure of Buna – S is given by:

Buna – S is formed using the monomers butadiene and styrene.

Thus in Buna – S, the Bu stands for Butadiene, since butadiene is one of the constituent monomers of the given polymer. Also, Na stands for sodium and S represents the monomer Styrene used for the synthesis of the given polymer.