It denotes to a generic sense, girl , boy, Dog, Building,Restaurant, it not represent specific thing or person common noun is every where.
Brands, government title are not consider as common noun
Examples of Common Noun:
A boy offers his father to go library with him.
A Teacher ask student for yesterday home work.
Common Noun not Capitalize if it appear in start of the sentence it will be capitalized.
Sunny got the master degree.
Venice is one of the best city in the world.
Toyota made very luxurious cars.
Mount Everest is one of the highest mountain.