This is the 805th most frequent Dutch word.
a
The Dutch word "a" does not have meaning on its own; it is not a standard word but could represent a letter of the alphabet.
In this sentence, 'een' is the Dutch equivalent of 'a/an' in English, used as an indefinite article to describe an unspecified singular item, in this case, 'apple'.
Een appel ligt op de tafel.
An apple is on the table.
Here, 'een' is functioning as an indefinite article to indicate an unspecified singular 'question'.
Ik heb een vraag voor de leraar.
I have a question for the teacher.
The word 'een' again serves as an indefinite article, describing an unspecified singular 'spot'.