What does saying "あ" mean in japan or any where?

I noticed this in a anime ~ Azumanga Daioh episode 8. "Can be found on youtube" They seem to say this in between either a change in view points or a change of scene in the form of a small cut scene. I understand this is a character of hiragana and what it leter it represents, Im just wondering if it has any special meaning or is it just something spontaneous. Thank you vary much to any one who can answer this for me.