This Latin phrase translates as:
Who will guard the guards themselves?
It was first found in the Satires of the Roman poet Juvenal. While this first usage was in the context of marital fidelity, modern use is more with regards to the checks and balances required of those who hold the power.
This is closely linked to the Paradox of Democracy, where the democratic process itself can be misused, with horrific consequences. It is easy to forget that Hitler came to power by democratic means and then used the Weimar Constitution against itself, especially the emergency power that it granted him.
The Paradox of Democracy can also happen when whoever holds the majority takes voting rights away from a minority. This process can repeat until only a tiny fraction of a population has the right to vote, at which point it is no longer a real democracy.
In a well-functioning democracy, the answer to who will guard the guards is quite simple: everyone.
This is why freedom of speech and independent bodies such as the judiciary and the press is essential — because they help keep politicians in check.