If checks and balances didn't exist, there would be nothing stopping a group i.e. the president, from making whatever laws they wanted. In essence, if there are no checks and balances, government is no longer a democracy because nothing prevents the group in power from doing what they want. For example, a president that wasn't checked by the opposition party could pass a law saying that they are legally allowed to be president until they die, and nobody would stop them.
The main way in which Jefferson does this is to use the phrase "we the people" when writing the Declaration of Independence, since this can be meant to mean the people of the entire world.
