Montesquieu's theory:

Montesquieu, a French philosopher of the 18th century, is often credited with the theory of separation of powers. According to Montesquieu's theory, “the powers of government should be divided into three separate branches: the legislative, the executive, and the judiciary. Each branch should have a distinct function, and each should be independent of the others, with no one branch having too much power or control.” Montesquieu believed that the separation of powers was essential for preventing tyranny and ensuring the protection of individual rights and liberties. He argued that if all power were concentrated in one person or group, there would be a greater risk of abuse of power and the violation of individual rights.

In Montesquieu's theory, 

the legislative branch is responsible for making laws,

the executive branch is responsible for enforcing laws, and

Each branch should be able to check and balance the others to ensure that no one branch becomes too powerful.