Long ago, nobles were people with wealth and power. Nobles weren't kings or queens, but they were favored by kings and queens and had a high ranking in society.
Because nobles had that high, lofty status in society, we've come to use the word "noble" to describe things that are high and lofty and impressive in other ways.
A noble person, or a noble action, is brave, honest, honorable, impressive, and helpful toward other people.
A noble knight might risk his life to slay a dragon and save a village. Noble teachers work long, unpaid hours and spend their own money on supplies so that kids can learn. And a noble friend will stand up to a bully for you.
If you have a noble goal or a noble plan, you want to help other people and make the world a better place. If you received money for your birthday, and you donate it to a food bank to help hungry people in need, that's a noble choice, and a noble way to spend your money.
Noble things and animals can also be grand, fancy, and beautiful.
Here's Shadowfax, a noble animal. He's proud, graceful, powerful, and beautiful. Check out his noble head, held high, and his noble stride, strong and sure.