The Big Lie is a propaganda technique whose invention is often falsely credited to Adolf Hitler (Hitler was not advocating the use of this technique but was in fact accusing Jews and Marxists of using it).
The basic idea is that the bigger a lie you tell, the more likely people will be to believe it, since it's difficult for them to accept that someone could tell a lie that huge with a straight face.
The Republicans keep pushing The Big Lie that the election was rigged. If anything, it was rigged in Trump's favor. President Biden was elected fairly and lawfully. He won 7 million more votes.