“A/An” articles are called “Indefinite” because they are:  not specific, new or unknown to your listener:

1. I need a new phone. (not a specific phone, just any phone)
2. Linda is an arquitect. (new to the listener you are speaking to)
3. You will find a tour guide somewhere. (unknown to the listener)

“The” articles are “definite” or specific articles. They are known to the listener:

1. The sushi at the restaurant you recommended was fantastic. (known to listener)
2. I prefer the blue car, not the red one. (specific)