This is the 101st most frequent French word.
mon
"Mon" means "my" (possessive adjective, singular, used before masculine nouns or nouns starting with a vowel sound).
Although 'amie' is a feminine singular noun, 'mon' is used instead of 'ma' because 'amie' begins with a vowel sound, thus ensuring ease of pronunciation.
Mon amie habite à Paris.
My (female) friend lives in Paris.
The word 'mon' is a possessive adjective referring to the masculine singular noun 'train', indicating that the speaker has a personal relation to it (my train).
Je monte dans mon train.
I am boarding my train.
The word 'mon' here is a possessive adjective used before the masculine singular noun 'frère' to indicate possession (my brother).