It is put at the beginning of verbs. But it doesn’t seem to change their meanings too much:
He came running to the store.
He came a’-running to the store.
On top of that, most writers don’t even use it any more. Does anyone know how exactly “a’-” changes the meaning of verbs, if in any way? I realize it does change the meaning a little. But what significance does it have today? And moreover how exactly was it originally used?
I always understood this to be verbal padding, used in songs to balance the number of syllables needed to rhyme or complete a line. For example, in Dylan’s “the times they are a-changin’”, removing the “a” would force the singer to pause or stretch out the vowel of some other word in order to fit the meter of the song. (In older English folk singing there is the similar -o ending, which always struck to me as a somewhat lazy way of making rhymes.)
Although I don’t think I have seen this construct used outside of song and poetry, it also applies a colloquialism in verbal speech. In your example there is no vowel between the words, and by padding with a neutral vowel such as “a”, the sentence becomes marginally easier to speak; in other words, it’s basically sloppiness. I don’t perceive there as being any change of meaning here.
It is also supposed to be a “folksy” way of saying things. L’il Abner used it a lot.
I don’t know if there is any real basis for the convention, though.
I’ve always read it to mean now, or immediately, so “He came a-running” is, “He came running immediately.”
The dictionary is your friend.