what does it mean c# is object-oriented

Answer 1

roughly speaking it means that we the programmer give more importance to objects(i.e. data) than to the logic(algorithm/procedure) of the program
with this concept come other concepts like data hiding, data encapsulation, data abstraction, modularity, polymorphism, inheritance, etc

Answer 2

Object orientation is an idea that is intended to make thinking about programming similar/closer to the real world. So essentially we think of most(if not all) things as objects in terms of programming . For more details visit the microsoft c# page.