In English there are two indefinite articles a and an. They are used before a noun to define it as something non-specific.
- I need to buy a new phone.
(I am not talking about a specific phone.)
- Do you want a cup of coffee?
(I am not thinking about a specific coffee.)
- This is my first time in an airplane.
(This is the first time in any plane.)
- I want an apple for my lunch.
(It can be any apple.)
Compare with the definite article.