Question

how is rule of law considered as the foundation of a democracy

Answer 1

Rule of Law and Democracy: Addressing the Gap Between Policies and Practices. ... If considered not solely an instrument of the government but as a rule to which the entire society, including the government, is bound, the rule of lawis fundamental in advancingdemocracy.

Answer 2

The concept of the rule of law isn’t widely understood. It means that the government has to act within the law. For example, even Donald Trump isn’t able to have his opponents locked up. Only a court of law can do that on conviction or a plea of guilty.
Without the rule of law, the legislature can legislate till it’s blue in the face, but if the laws only apply to ‘ordinary folk’ and not to the government it’s all rather pointless. The rule of law is the only guarantee against arbitrary government.