The meaning of articles a & an.

a box	an apple

The twentysix alphabets of English language are divided into vowels and consonants.

A,e,i,o,u are vowels. All the other alphabets are consonants.

'A' is used when a word begins with a consonant. eg a pen. The letter 'p' is a consonant .

'An' word used when a word begins with a vowel. eg an orange. The letter 'o' is a vowel.

The usage of a or an also depends on the sound of the words.
For eg an heir . Although the letter 'h' is a consonant, we use the article 'an' because theconsonant 'h' is silent.

Similarly, we also say a university. Although the letter 'u' is a vowel, we use the article 'a' because the word university begins with the consonant sound 'yu'.