A principle is a rule or code of conduct.
From the time you were old enough to understand, your parents began to teach you such principles: for example, Tell the truth. Don't steal. Respect your parents. These and other principles should go together to form your own personal code of conduct. The Ten Commandments are a code of conduct.
My four favorite presidents all had a code by which they lived, and they stuck to their values even though others put much pressure on them to give up their principles for the sake of popularity. They all knew better.
George Washington actually wrote or copied a code of conduct for himself when he was about sixteen years old. He was throughout his life conscious of how he behaved, what causes he would support and how he would treat others. When he became president, he was very careful about his acts, because he knew that whatever he did, he would set a precedent for the presidents who followed.
The three other presidents were Abraham Lincoln, Theodore Roosevelt, and Harry S Truman. Although I was a young boy and didn't know much about politics when Mr. Truman was president, I do remember how much animosity he collected during his term of office. It was prevalent; especially, when he fired General Douglas MacArthur. Now people think he was one of the truly great presidents, because he made tough decisions, acting on what he believed was right, not on what would make him popular.
There is going to come a time in your life, when you will have to make tough decisions for which you will have to choose between maintaining your principles or trading them for something like popularity or even a job. Stand by your principles. People may not agree with your position, they might not even like you, but they will respect you. Even more than that, you will respect yourself. It is a good thing to be able to look at yourself in the mirror in the morning and know that you did the right thing.
Beware. If you ever decide to trade your principles for popularity, you will have neither respect nor popularity.
I suggest and encourage you to read biographies of those four presidents.