This is the 61st most frequent Lithuanian word.
a
The Lithuanian word "a" is an interjection used to express surprise, realization, or attention, similar to "oh" or "ah" in English.
The word 'a' here indicates an indefinite, singular noun in the translation.
Aš mačiau ežerą.
I saw a lake.
The word 'a' in the translation is part of the phrase indicating a single unspecified friend.
Tai buvo sunku, a draugas man padėjo.
It was hard, but a friend helped me.
In Lithuanian 'A' is the starting syllable of the word 'Ačiū,' which means 'Thanks.' It's unrelated to usage in English.