mio and el mio
Here is a reference article that should help to answer your question. Feel free to come back if there is something you still don't understand. Possessive Pronouns
mío and mía are long form possessive adjectives.
libro mío=my book
manzana mía=my apple
el mío and la mía are possessive pronouns as Nicole says.
Ese libro es (el) mío. [the definite article is dropped after the verb Ser] That book is mine.
Esa manzana es (la) mía. That apple is mine.