{tikz-chains} concerns the TikZ-pgf library chains
The tikz-chains concerns the TikZ-pgf library chains. Chains are nodes arranged in sequence and connected by edges. A chain is thus a description of how the nodes are connected. An example from the TikZ-pgf manual:
\begin{tikzpicture}[start chain]
% The chain is called just "chain"
\node [on chain] {A};
\node [on chain] {B};
\node [on chain] {C};
\end{tikzpicture}