Vivitar Serries I 600 M/P/O

[Zalerk]

I'm a bit of a newby to photography, so wondered if anyone could explain to me what the different modes on this flash I've recently acquired actually mean.

Everything is fine, apart from one switch which has four options: TTL, A1, A2 and M. Changing between these different modes provided me with a different range and f/stop. So in what instance do I use each of these?

Thanks.