The Constitution divided our government into 3 branches: legislative, executive, and judicial. A system of checks and balances ensures that even though there is a separation of power between the 3 branches, they work together to ensure there is not too much power in one branch.
Congress creates the laws, processes a bill, and then sends it to the president to sign into law or veto and send back to the Congress to vote again. The judicial branch reviews the law and decides if it constitutional or not.