To what does the phrase checks and balances refer?

Answer 1

Checks and Balances is the means whereby each branch of government has some control on the other branches. Congress, for example, may pass a law but the President may check Congress and reject the law by his veto power. The Congress, in turn, can check the President by overriding the veto by a two-thirds vote of each house. The federal courts can check the executive and the legislative branches.

the checks and balances is the 3rd most important part of the constitution

Answer 2

It refers to the balance of power between the three branches of the federal government which are the legislative (congress) the executive (president) and the judicial (supreme court). The idea is to not have too much power in any one branch thereby maintaining checks and balances against each of the other branches.