To what does the phrase checks and balances refer?

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