A floor is a horizontal surface inside a building. People walk on a floor. A floor can be made of many different materials, including concrete, tiling, or wooden floorboards. People often put carpets on a floor.
The word "floor" can also mean the level of rooms in a building. Another word for this is "storey".
The way these levels are numbered may be different in different countries. In Britain and most of Europe, the ground level floors are called the "ground floor", the next level is called the "first floor". In most of the United States, Russia and China, the floor at ground level is called the "first floor", the next level is called the "second floor". This can be confusing for people visiting other countries.