With parts of sizes \(a\) and \(6-a\) there is room for \(a(6-a)\) edges, maximal at \(a=3\): \(3\cdot3=9\). The 15 edges of \(K_6\) would include triangles.
A bipartite graph is a graph \(G=(V,E)\) whose vertices split into two disjoint sets \(A\) and \(B\) so that every edge \(e\in E\) joins a vertex of \(A\) to one of \(B\).
\[V=A\cup B,\quad A\cap B=\varnothing,\quad |e\cap A|=1\ \text{ for every } e\in E\]