What is meant by the Domino Theory?

It depends on what you're referencing it to. the domino theory, in one historical aspect, was referenced during the Vietnam war. It was meant to say that if the US lost Vietnam to the communists, then countries like Cambodia, Burma, and eventually India would fall to the communists.

The Domino Theory was a theory prominent from the 1950s to the 1980s that posited that if one country in a region came under the influence of communism, then the surrounding countries would follow in a domino effect.