This is the 5th most frequent French word.
a
"Has" (third person singular of "avoir").
In this case, 'a' is the third person singular present tense form of the verb 'avoir' (to have).
Il a un chien.
He has a dog.
In this sentence, 'a' is used to describe a characteristic or fact about the subject, specifically the composition of the word.
Le mot 'chat' a trois lettres.
The word 'chat' has three letters.
Here, 'a' again signifies possession or existence, as in 'the house possesses a red roof'.