Something that's grand is large and impressive. A fancy, catered birthday party with a guest list of hundreds could be described as grand.
The adjective grand can be used in many different ways: it can describe big things, like a grand piano or the Grand Canyon, or luxurious things, like the grand leather seats in your dad's Cadillac. You can also use grand to simply mean "wonderful," as when you exclaim, "Chocolate cake for dessert! Isn't this grand!" Grand is also a slang term for "a thousand dollars." The Latin root is grandis, "big, great, full, or abundant."