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.