A or An are used before countable nouns to talk about something the listener or reader does not know about.
- I was born in a town.
(The reader doesn’t know which town and no vowel sound so a.)
- They are a company which sells chocolate.
(Lots of companies sell chocolate so, the listener doesn’t know which company is being talked about and no vowel sound so a.)
- I want to buy an umbrella.
(There are lots of umbrellas to buy, but we don’t know which one. Here there is a vowel sound, so we use an.)