Most adjectives are "common adjectives". They describe people, places and things, for example:
1. tall man, big dog, noisy baby
2. large garden, empty room, beautiful city
3. red car, hot weather, expensive watch

"Proper adjectives" also describe people, places and things, but they are based on names and therefore need to be capitalized, like the names.
For example :
1. Spain is the name of a country and starts with a capital letter. The adjective based on Spain (Spanish) must also start with a capital letter.
2. Mars is the name of a planet and starts with a capital letter. The adjective based on Mars (Martian) must also start with a capital letter.