“a” is a word and letter in English with several meanings:

• Indefinite article: used before a word that starts with a consonant sound
  • Example: a car, a book
• Letter: the first letter of the English alphabet
  • Example: A, B, C…
• Mathematics/science: sometimes used as a variable or symbol
  • Example: a + b = c

In short, “a” is a basic English article and also the first letter of the alphabet.