- Congress is one of three equal branches of government
- Legislative branch makes laws, executive implements enforces them, judicial branch interprets them
Congress makes the laws.
If you only know one thing about the legislative branch, that should be it. (Now, it may or may not be a good idea to know more than one thing, but that's up to you.)
Congress makes the laws, and that makes Congress the heart of America's representative democracy—the place where the democratic representation actually happens, where lawmakers elected directly by the people meet to make laws to (hopefully) serve the public interest.
There's a reason why founding father James Madison called Congress "the first branch" of government. Under the Articles of Confederation—America's first independent system of government, which lasted from 1781 to 1789—the national government consisted only
of Congress. There was no president and no federal court system, no executive or judicial branches of government. That approach didn't work out very well, which is why 55 leading citizens—including legendary figures like Madison, George Washington, Benjamin Franklin, and Alexander Hamilton—met in Philadelphia over the summer of 1787 to hammer out a new arrangement for national government. That new arrangement, the Constitution of the United States, created a much more robust government, one in which Congress's powers were checked and balanced by newly created executive and judicial branches.
Even so, the legislative branch remained at the center of the framers' vision for representative democracy. It's no accident that the legislative branch is described in Article I of the Constitution, or that Article I is longer than the other six articles combined. A large majority of the Constitutional Convention's time and energy was poured into resolving thorny problems in the design of the legislative branch. By comparison, the presidency (Article II) and the federal court system (Article III) were tacked on as relative afterthoughts.
Under the new system of government created by the Constitution—the system we still use today—the legislative branch lost its monopoly on power and became one of three equal branches of government. Still, the legislative branch remained something of a first among equals; the Constitution empowers the president to execute the laws and the judiciary to interpret them, yet only Congress makes the laws.